In the Name of Allah, the Most Merciful, the Bestower of Mercy
Ibn Abil-’Izz (d.792H) - rahimahallâh - said:
“Knowledge of Usûlud-Dîn (the fundamentals of the Dîn) is the most noble branch of knowledge, since the excellence of a certain type of knowledge depends upon what it is concerned with, and this is the greater Fiqh (understanding), which is why Imâm Abû Hanîfah (d. 150H) - rahmatullâhi ’alayhi - called that which he compiled concerning Usûl ud-Dîn: “al-Fiqhul-Akbar.” (the Greatest Fiqh). The need of the servants for this knowledge is greater than every other need; and it is the most necessary of all things for them, since there is no life for the hearts, nor any delight, nor any tranquility, except through knowing their Lord, the One to be worshipped, their Creator - with His Names, His Attributes and His Actions, and that He - along with all that - is more beloved to the person than anything else. So man’s striving is with regards to everything that will draw him nearer to Allâh, to the exclusion of the creation.
However, it is impossible for the minds to come to know and understand all that in detail, so the Most Merciful , the Most Majestic - from His mercy, sent Messengers to teach that and call to it; and to give good news to those who accept their Call and to warn those who reject it. The key to their Call and the essence of their message was the servant’s drawing closer to Allâh - the Most Perfect - through His Names, Attributes and Actions, since all that the Messengers were ordered with is built upon this. This then is followed by two great principles:- Firstly: Knowing the path that leads to Him - and that is the Sharî’âh which is comprised of His orders and prohibitions. Secondly: That those following the path know what lies in store for them, which is endless bliss. So the people who know Allâh best are the ones who best follow the way to Him; and know best what lies at the end of the way.” 
Firmness Upon the Dîn
So, firmness upon the Dîn of Allâh, excellence in this world and salvation in the Hereafter is built upon two great matters: “Firstly: Knowledge of Allâh and the beautiful Names and lofty Attributes that befit Him and His Actions - and this necessitates appreciation of His Majesty, honouring Him, fearing Him, being in awe of Him, loving Him, placing one’s hopes in Him, placing reliance upon Him, being pleased with His decree and having patience with what He sends down as regards hardships. Secondly: Knowledge of what He loves and is pleased with, and what He hates and angers Him - whether beliefs saying, or outward or inward actions. So the one who has knowledge of this has to rush to fulfill that which Allâh loves and is pleased with, and to avoid that which He hates and which angers Him.” 
Sufyân ibn ’Uyaynah (d.197H) - rahimahullâh - said:
“There are three types of Scholars: one who knows Allâh and knows Allâh’s commands; and one who knows Allâh, but does not know His commands; and one who knows Allâh’s commands, but does not know Allâh. And the most complete of them is the first - and that is the one who fears Allâh and knows His rulings.” 
The Essence of Islâm
Explaining the essence of Islâm and its main pillar, the Prophet sallallâhu ’alayhi wa sallam said: “Islâm is built upon five: Testifying that none has the right to be worshipped except Allâh and that Muhammad is the Messenger of Allâh, establishing the Salâh, paying the Zakâh, making pilgrimage to the House and fasting in Ramadhân.”  In another narration: “Islâm is built upon five: To worship Allâh and to reject anything along with Him ... ”  Also in another narration: “Islâm is built upon five: The Tawhîd of Allâh ... ” 
Thus: “Testifying that none has the right to be worshipped except Allâh,” has the same meaning as: “To worship Allâh and to reject anything along with Him,” which has the same meaning as: “The Tawhîd of Allâh,” So, it will be clear to the honourable reader that Tawhîd is the essence of Islâm, and it is the starting and ending point for all goodness and excellence.
Linguistically Tawhîd means: “To make something one, or to assert the oneness of something.”  However, what we are concerned with here is the Sharî’ah or technical meaning of Tawhîd which is: “To single out Allâh alone for worship.” 
Al-Bayjowrî - rahimahullâh - said:
“It is to single-out al-Ma’bûd (the One to be worshipped - i.e. Allâh) with worship, along with belief and affirmation in the oneness and uniqueness of His Dhât (Essence), Sifât (Attributes) and Actions.” 
Shaykh al-Ghunaymân - hafidhahullâh - said:
“It is to single Him out with worship, with love, lowliness and submissiveness to Him, by complying with His commands and submitting to them.” 
The Divisions of Tawhîd
And Tawhîd - with the Salaf and the Scholars of Ahlus-Sunnah wal-Jamâ’ah - has three divisions.
’Allâmah as-Safârînî (d.1112H) - rahimahullâh - said:
“Know that Tawhîd has three divisions:- Tawhîd ar-Rubûbiyyah (the Oneness of Allâh in His Lordship), Tawhîd al-Ulûhiyyah (to single-out Allâh alone for worship) and Tawbîd al-Asmâ was-Sifât (the uniqueness of Allâh’s Names and Attributes).” 
“And Allâh has gathered these three divisions in His - the Most High’s - saying: “Lord of the heavens and the earth and all that is between them, so worship Him alone and be constant and patient in the worship of Him. Do you know of any who is similar to Him?” ” 
“So Tawhîd ar-Rubûbiyyah implies: A firm and definite belief that Allâh alone is the Creator, the Master and Owner, and the Command is for none but Him.” 
“And Tawhîd al- Ulûhiyyah is to single-out Allâh alone for all worship and not to worship anything along with Him, whether it be an angel, a Messenger, a Prophet, a pious person, a tree, a stone, the sun, the moon, or other than these.” 
“And Tawhîd al-Asmâ was-Sifât is the uniqueness of Allâh - the Most High - with regards to His Names and His Attributes, by affirming that which Allâh has affirmed for Himself - whether in His Book or by the tongue of His Messenger sallallâhu ’alayhi wa sallam - without tahrîf (distorting the Names and Attributes), ta’tîl (denial of the Names and Attributes), takyîf (saying how they are), or tamthîl (making any resemblance with the creation).” 
Tawhîd in Knowledge and Action
The above three divisions of Tawhîd have been grouped - by some of the Scholars - into two types:- the first type deals with knowing Allâh through His Names, Attributes and Actions (i.e. Tawhîd al-Asmâ was-Sifât and Tawhîd ar-Rubûbiyyah), and the second deals with actualising and manifesting this Tawhîd through singling out Allâh alone for worship (i.e. Tawhîd al-Ulûhiyyah). The first type of Tawhîd is connected with knowledge, whilst the second type is connected with action.
Ibn al-Qayyim (d.756H) - rahimahullâh - said:
“As regards the Tawhîd which the Messengers called to and which the Books were sent down with, then it is of two types:- Tawhîd al-Ma’rifah wal-Ithbât (the Tawhîd of knowledge and affirmation) and Tawhîd fit-Talab wal-Qasd (the Tawhîd of actions and intentions).
So the first type affirms the reality of the Dhât (essence) of the Lord - the Most High - along with His Names, His Attributes, His Actions, His speaking in His Books and His speaking to whomsoever He wishes from His servants. It also affirms the all-embracing nature of His Predestination and Pre-Decree and His wisdom. The Qur‘ân has completely clarified this type of Tawhîd - as occurs at the start of Sûrah Hadîd and Sûrah Tâ Hâ, at the end of Sûrah al-Hashr, at the beginning of Sûrah Sajdah, at the beginning of Sûrah Âl-’Imrân and all of Sûrah al-Ikhlâs, and other than these.
The second type: Then it is what is contained in Sûrah al-Kâfirûn and contained in His - the Most High’s - saying: “Say: O people of the Book! Come to a word that is just and fair between us and you, that we shall worship none but Allâh and that we shall associate no partner along with Him, and that we shall not take others as lords beside Allâh. Then if they turn away, say: Bear witness that we are Muslims.”  It is also contained at the beginning of Sûrah Tanzîl and at the beginning, middle and end of Sûrah al-Mu’min, and at the beginning and end of Sûrah al-A’râf and the greater part of Sûrah An’âm. Indeed every Sûrah in the Qur‘ân comprises these two types of Tawhîd, witnessing to it and calling to it.
Since the Qur‘ân either gives [i] information about Allâh, His Names, His Attributes, His Actions and His Sayings - which is Tawhîd al-Ma’rifah wal-Ithbât (the Tawhîd of knowledge and affirmation); or [ii] it is a call to worship Allâh alone without any partner, and a rejection of whatever is worshipped besides Him - and this is Tawbîd al-Irâdî at-Talabî (the Tawhîd of actions and intentions); or [iii] it is a command to obey Him and to comply with His orders and prohibitions - so this is from the rights of Tawhîd and is a completion of it; or [iv] it gives information about the people of Tawhîd and how they were treated in this world and how they will be honoured in the Hereafter - and this is the reward for Tawhîd; or [v] it gives information about the people of Shirk (associating partners along with Allâh) and the punishment they receive in this world and the torment they shall receive in the Hereafter - so this is the reward for those who abandon Tawhîd.” 
Tawhîd - its Importance in the Qur‘ân
Ibn Abil-’Izz - rahimahullâh - said:
“So the Qur‘ân - all of it - is about Tawhîd, its rights and its rewards; and about Shirk, its people and their punishments. Thus: “All praise belongs to Allâh, Lord of the worlds.”  is Tawhîd (of knowledge). “The Most Merciful, the Bestower of Mercy.” is Tawhîd (of knowledge). “The Master of the Day of Judgement.” is also Tawhîd (of knowledge). “You alone do we worship and You alone do we seek aid and assistance from.” is Tawhîd (of action). “Guide us to the Straight Path.” concerns Tawhîd and asking for guidance to the path of the people of Tawhîd, (which is): “The Path of those whom You have favoured. Not of those who have earned Your anger, nor of those who have gone astray.” those who have separated themselves from Tawhîd.”  So Allâh begins this Sûrah by first informing us about Himself - the Most Perfect (i.e. Tawhîd of knowledge). Then after having knowledge of Allâh - the Most High - Allâh orders us with the Tawhîd of action: “You alone do we worship.”
Likewise just as: “the great Qur‘ân opens with Tawhîd it also ends with it. So the Qur‘ân opens with Sûrah al-Fâtihah: “All praise belongs to Allâh, Lord of the worlds.” and the great Qur‘ân ends with the Sûrah: “Say: I seek refuge with the Lord of mankind.” ” 
Likewise, the greatest âyah in the Qur’ân is Âyatul-Kursî.  Since this great âyah informs us - from start to finish - purely about Allâh’s Names, His Attributes and His Actions. So this further emphasises the importance of the Tawhîd of knowledge
Similarly, Sûrah al-Ikhlâs - from start to finish - is concerned with the Tawhîd of knowledge, whilst Sûrah al-Kâfirûn is connected with the Tawhîd of action. Indeed, the Prophet sallallâhu ’alayhi wa sallam said about a man who was reciting al-Kafirûn in the first rak’ah: “This is a servant who believes in his Lord.” Then the man recited al-Ikhlâs in the second, so he sallallâhu ’alayhi wa sallam said: “This is a servant who knows his Lord.”  The Prophet sallallâhu ’alayhi wa sallam said: “Say: He is Allâh, the One” is equivalent to one third of the Qur‘ân. And “Say: O disbelievers ... ” is equivalent to a quarter of the Qur‘ân. 
What further demonstrates the importance of this Tawhîd is the fact that the Prophet sallallâhu ’alayhi wa sallam “would begin his day with Tawhîd (of knowledge and action), since he recited both Sûrah al-Kafirûn and Sûrah al-Ikhlâs in the two rak’ât before Fajr (dawn prayer).  And he would end the night by reciting both of these Sûrahs in his witr prayer. ” 
“Indeed, all of the Qur‘ân is about Tawhîd.”  And Allâh - the Mighty and Majestic - says: “A book We have sent down, full of blessings, that men may ponder over its messages; and those who possess understanding may take them to heart.” 
Tawhîd - its Importance from the Sunnah
And what proves the importance of Tawhîd from the Sunnah is that the Chosen one - ’alayhis-sallâtu was salâm - remained in Makkah thirteen years amongst the disbelievers, calling them to it, and saying to them: “Say: None has the right to be worshipped except Allâh and thus be successful ... ” 
The Messenger of Allâh sallallâhu ’alayhi wa sallam said: “I have been sent close to the Hour, with the sword, so that none has the right to be worshipped except Allâh alone, without any partner. My sustenance is beneath the shade of my spear. And humiliation and ignominy is for whosoever opposes my command. And whosoever resembles a people is from them.” ” 
Similarly he would send his Companions to various communities ordering them with this Tawhîd first - as was the case when he sent Mu’âdh ibn Jabal to Yemen, saying: “Indeed you are going to a people from the People of the Book, so let the Tawhîd of Allâh be the very first thing you call them to ... ” 
And just as the Prophet sallallâhu ’alayhi wa sallam started his Prophethood and teaching with the matter of Tawhîd, then likewise, “he sallallâhu ’alayhi wa sallam mentioned the matter of Tawhîd during his final illness from which he ’alayhis-sallâtu was salâm died, when he said: “The curse of Allâh be upon the Jews and the Christians, for they took the graves of their Prophets as places of worship.” ” 
Clearing a Misconception
Shaykhul-Islâm Ibn Taymiyyah (died 728H) said:
“The Tawhîd with which the Messengers came with comprises of affirming that Divinity and worship belong to Allâh alone, such that a person witnesses that none has the right to be worshipped except Allâh, and that none is worshipped except Him, nor depended upon other than Him, nor are alliances or enemies made except for Him, nor is an action done except for Him. This affirmation also covers those Names and Attributes which Allâh affirms for Himself, as Allâh - the Most High - says: “And the Allâh to be worshipped is only one Allâh. None has the right to be worshipped except Him, the Most Merciful, the Bestower of Mercy.”  And Allâh - the Most High - says: “Do not take two objects of worship. Indeed, Allâh is the only Deity to be worshipped, so fear Him.” . And He - the Most High - says: “Whosoever calls upon (i.e. supplicates or prays to) other than Allâh, for which he has no proof, then his reckoning is only with his Lord. Indeed the disbelievers will not be successful.”  Allâh - the Most High - also says: “And ask those of our Messengers whom we sent before you: ‘Did We ever appoint objects of worship besides the Most Merciful?’ ” 
And Allâh informed us about every Prophet from amongst the Prophets that they called the people to the worship of Allâh alone, having no partner, as He said: “Indeed there is an excellent example for you in Abraham and those with him, when they said to their people: ‘Indeed we are free from you and whatever you worship besides Allâh. We have rejected you, and there has arisen between us and you, hostility and hatred forever - until you believe in Allâh alone.’ ”  And Allâh said about the mushriks (pagan idolaters): “When it is said to them: ‘None has the right to be worshipped except Allâh’, they become arrogant. And they say: ‘Are we to abandon those deities that we worship because of a mad poet?’ ”  And this occurs a lot in the Qur‘ân.
And what is meant by Tawhîd is not merely Tawhîd ar-Rûbubiyyah - the belief that Allâh alone is the Creator - which is what some of the people of Kalâm (speculative theology) and the Sûfîs think! So they think that if they affirm this type of Tawhîd, along with its proofs, then they have affirmed the utmost limits of Tawhîd and that if they bear witness to this and become absorbed in it, then they have absorbed themselves in the limits of Tawhîd! However, this is not the case. Since even if a person agreed to those attributes which are befitting to the Lord, and declared Him free from everything He should be declared free from and affirmed that He alone is the creator of everything - then still such a person would not be a muwahhid (a person of Tawhîd) until along with this, he witnesses that the only Ilâh (i.e. object of worship) is Allâh (Allah) - (i.e. none has the right to he worshipped except Allâh alone) - affirming that Allâh alone is the Ilâh deserving off all worship, adhering to this worship and associating no partners with Him. Also affirming that this Ilâh (object of worship) is He who is deified and worshipped and who deserves worship and it is not that Ilâh merely with the meaning: “The One who has the power to create and originate.” 
So if an explainer (of the Qur‘ân) explains Ilâh to mean: “The One who has the power to create and originate” and believes that this is the most particular description of Ilâh and affirms this to be the limits of Tawhîd - as is done by the people of Kalâm, and it is what they say from Abûl-Hasan al-Ash’arî  and his followers - then they do not know the true reality of the Tawhîd with which Allâh sent His Messengers, since the Arab mushriks used to agree that Allâh alone is the Creator of everything. Yet despite this, they were still mushriks, as Allâh - the Most High - says: “Most people do not believe in Allâh except that they commit shirk (i.e. associate others with Him in belief and worship).” 
A group from amongst the Salaf (the first three generations of Muslims) said: “If you ask them who created the Heavens and the earth, they will say: ‘Allâh’ - yet along with this, they worshipped others besides Him.” 
Allâh - the Most High - said: “Say: ‘Whose is the earth and all that is in it, if you do indeed know?’ They will say: ‘It belongs to Allâh.’ Say: ‘Will you not then remember?’ Say: ‘Who is the Lord of the heavens and the Lord of the great Throne?’ They will say: ‘Allâh.’ Say: ‘Will you not then fear Him?’ ”  So not everyone who affirms that Allâh is the Lord of everything and is its Creator, will be a worshipper of Him to the exclusion of everything else - calling upon Him alone, hoping in Him alone, having fear of Him alone, forming allegiance and enmity for Him, obeying His Messengers, ordering what He orders and forbidding what He forbade.” 
Shaykh ’Abdul-Qâdir al-Jîlânî (d.561H) said:
“So it is upon you to fear Allâh - the Mighty and Majestic - and not to fear anyone else except Him. Turn to Allâh - the Mighty and Majestic - for your every need, and rely upon Him alone - the Most High - seeking that which you need from Him alone. Do not rely upon anyone other than Allâh. And Tawhîd - all is contained in Tawhîd.” 
1. Sharhul-’Aqîdatul-Tahâwiyyah (p.69).
2. Fadl ’Ilmus-Salaf (p.47) of al-Hâfidh Ibn Rajab (d.795H).
3. Related by ad-Dârimî (1/102) and Abû Nu’aym in al-Hilyah (7/280), with a Sahîh isnâd.
4. Related by al-Bukhârî (1/49) and Muslim (no. 16) from ’Abdullâh ibn ’Umar - radiallâhu ’anhu.
5. Related by Muslim (no.20).
6. Related by Muslim (no.19).
7. Lisân ul-’Arab (3/450) of Ibn Mandhûr and also al-Hujjah fî Bayânil-Mahajjah (1/305) of Abûl-Qâsim al-Asbahânî.
8. Ad-Dururus-Sunniyyah (1/48) of Shaykh ’Abdur-Rahmân ibn Hasan.
9. Jawharah at-Tawhîd (p. 10).
10. Sharh Kitâbut-Tawhîd min Sahîhil-Bukhârî (1/38).
11. Lawâmi’ al-Anwârul-Bahiyyah (1/128) of as-Safârînî. For the division of Tawhîd into three types, refer to: al-Ibânah ’an Sharî’atil-Firqatin-Nâjiyah (p. 693-694) of Ibn Battah (d.387H); Kitâb ut-Tawhîd of Ibn Mandah (d.395H) and al-Hujjah fî Bayânil-Mahajjah (1/85, 1/111-113) of Abûl-Qâsim al-Asbahânî (d.535H).
12. Sûrah Maryam 19:65
13. Taqrîbut-Tadmuriyyah (p.110) of Shaykh Ibn al-’Uthaymîn.
14. Taqrîbut-Tadmuriyyah (p.110-111).
15. Taqrîbut-Tadmuriyyah (p.112-113).
16. Taqrîbut-Tadmuriyyah (p.116-117).
17. Sûrah Âl-’Imrân 3:64
18. Madârijus-Sâlikîn (3/449-450) of Ibn al-Qayyim.
19. Sûrah al-Fâtiha 1:1 - and what follows is a completion of this Sûrah.
20. Sharhul-’Aqîdatul-Tahâwiyyah (pp. 89-90) of Ibn Abil-’Izz.
21. Sûratun- Nâs 114:1
22. Hukmul-Intimâ (p.58) of Shaykh Bakr Abû Zayd - slightly adapted.
23. Related by Muslim (no.1768) from ’Ubayy ibn Ka’b - radiallâhu ’anhu.
24. Refer to Majmû’ al-Fatâwâ (1/54) of Ibn Taymiyyah.
25. Hasan: Related by at-Tahawî and Ibn Hibbân. Al-Hâfidh Ibn Hajar authenticated it in Ahadîthul-’Âliyât (no.16).
26. Sahîh: Related by at-Tabarânî in al-Mu’jamul-Kabîr (3/203/2) from Ibn ’Umar - radiallâhu ’anhu. It was authenticated by al-Albânî in Sahîhul-Jâmi’ (no. 4405).
27. Related by Muslim (no.726)
28. Sahîh: Related by an-Nasâ’î and al-Hâkim, who declared it sahîh.
29. At-Tawhîd wa Atharahu fî Hayâtil-Muslim (p.30) of Ahmad ibn Ibrâhîm al-Harîqî.
30. Madârijus-Sâlikîn (3/450).
31. Sûrah Sâd 38:29.
32. Related by Ahmad (4/63).
33. Sahîh: Related by Ahmad (no. 5114) and Ibn Asâkir (19/96/1), from Ibn ’Umar - radiallâhu ’anhu. It was authenticated by al-Hâfidh al-’Irâqî in Takhrîjul-Ihyâ (3/42), and al-Hâfidh Ibn Hajr in in Fathul-Bârî (10/222).
34. At-Tawhîd wa Atharahu fî Hayâtil-Muslim (p.29).
35. Related by al-Bukhârî (1/13) and Muslim (1/272), from Ibn ’Abbâs - radiallâhu ’anhu.
36. Related by Bukhârî (1/532) and Muslim (5/16).
37. At-Tawhîd wa Atharahu fî Hayâtil-Muslim (p.29).
38. Sûrah Âl-’Imrân 3:163.
39. Sûrah Nahl 16:51.
40. Sûrah Mu’minûn 23:117.
41. Sûrah az-Zukhruf 43:45.
42. Sûrah al-Mumtahinah 60:4.
43. Sûrah Sâfât 37:35-36.
44. This is the saying of the people of Kalâm, such as Abû Mansûr al-Mâturîdî in at-Tawhîd (pp.20-21), and for a similar misconception refer to Fî Dhilâlil-Qur’ân (5/2707) of Sayyid Qutb. Compare this meaning of Ilah with the correct explanations of great mufassirs - such as Imâm Ibn Jarîr at-Tabarî in his Tafsîr (20/102) and Ibn Kathîr in his Tafsîr (3/398) - which is: the one who is who is worshipped and who alone deserves to be worshipped.
45. He is Abûl-Hasan ’Alî ibn Ismâ’îl al-Asharî (died 324H) - to whom the Ash’ariyyah ’aqîdah is is incorrectly ascribed, since he abandoned this ’aqîdah - and that of the Mu’tazilah previously - for the ’aqîdah of the Salaf - as is mentioned by Ibn Kathîr in Tabaqâtush-Shâfi’iyyah and shown by his last book: Al-Ibânah ’an Usûlid-Diyânah.
46. Sûrah Yûsuf 12:106
47. This is the saying of lbn ’Abbâs and others - as occurs in Jâmi’ul-Bayân ’an Ta’wîlul-Qur‘ân (13/50-51) of at-Tabarî.
48. Sûrah Muminûn 23:84-87.
49. Majmû’ul-Fatâwâ (3/97-105) of Ibn Taymiyyah.
50. Futûhul-Ghayb (p.176).