This article is taken from the beginning of Shaikh ul-Islam's (r) treatise "
Raf'-ul-Malâm 'an-il-A'immat-il-A'lâm (Removing the Harms from the Noble Imâms)".
In this treatise, the Shaikh (ra) discusses the reasons why the famous and righteous Imâms, that are followed by the masses of Muslims, differed in some issues and contradicted the established ahâdîth of the Messenger of Allâh(swt) in some cases. He lists over ten reasons in the introduction of this treatise. However this article only contains the first one. However, it is the most important one, as stated by Ibn Taimiyyah (ra). And it is for this reason, as the Shaikh says, that many of the Imâm's opinions, that contradict the Sunnah, can be explained and acknowledged. Thus, there is no deficiency on the part of our scholars. Rather, they were rewarded in their correctness as well as in their errors. May Allâh have mercy on them. This article was translated by Isma'îl ibn al-arkân. The notes where taken from Zuhair Ash-Shâwîsh's comments and footnotes to this treatise, unless otherwise specified.
All praise is due to Allâh for His countless blessings. I bear witness that none has the right to be worshipped except Allâh, He stands alone and with no partners, whether on His earth or in His heavens. And I bear witness that Muhammad(saw) is His slave, Messenger and the last of His prophets. May Allâh send His eternal peace and continuous blessings upon him, his family and his companions until the Day when we shall all meet Him.
It is binding upon the Muslims, after declaring loyalty to Allâh and his Messenger, to declare loyalty to the believers as is stated by the Qur'ân. This especially includes the scholars, for they are the inheritors of the prophets. And they are the ones whom Allâh has given the status of bearing the similitude of stars, through which guidance is achieved in times of darkness, on land and on sea. The Muslims of this nation have agreed upon their guidance and their knowledge.
Before the advent of our Prophet, the scholars of every nation were the worst of its people. But this is not the case with the Muslims, for indeed their scholars are the best from among them. They are the delegated successors of the Messenger (saw) in regards to his nation and they are the ones who revive what has been caused to die from his (saw) Sunnah. It is through them that the Qur'ân is established and through it, they are established. Through them the Qur'ân is articulated and through it, they are articulated.
It must be known that there is not one scholar, who is accepted widely and willfully by the ummah, that purposely intends to oppose the Messenger of Allâh(swt) in any aspect of his (saw) Sunnah, whether small or large. For indeed, they all agree on the obligation of following the Messenger(saw) upon a firm and certain understanding. They also agree that one can accept or reject the statement of any individual from mankind, except that of the Messenger of Allâh. 
Therefore when it is found that an opinion, held by one of them, contradicts an authentic hadîth, there is no doubt that excuses must be made for him, in explanation to his abandoning of it. These excuses can be divided into three categories:
1. The lack of his believing that the Prophet s said it
2. The lack of his believing that particular issue was affected by that statement
3. His belief that that ruling was abrogated
These three categories can be subdivided into numerous reasons:
In spite of this, when Abu Bakr was asked on the inheritance of the grandmother, he said:
So he asked them and Al Mughîrah Ibn Shu'bah and Muhammad Ibn Maslamah stood up and testified
This aspect of the Sunnah was conveyed to 'Imrân Ibn Husain, also. These three individuals were not equal to Abu Bakr nor to any of the other (four rightly guided) khalîfahs in terms of knowledge. However, they were specifically characterized with knowledge of this Sunnah – of which its acting upon has been agreed on by the ummah.
Likewise, 'Umar did not know the Sunnah for asking permission to enter a household, until he was informed of it by Abu Mûsâ Al-Ash’arî, who called on the Ansâr as witnesses.  This was even though 'Umar possessed more knowledge than the one who told him of this aspect of the Sunnah.
Also, 'Umar did not know that a woman was supposed to inherit from her (deceased) husband's blood money, but instead he held the opinion that the blood money belonged to the 'آqilah. This was until Ad-Dahhâk Ibn Sufyân Al-Kulâbî, who was a leader of some Bedouin Arabs (Al-Bawâdî) for the Messenger of Allâh, wrote to him and informed him that:
So he abandoned his opinion in favor of that (hadîth) and said:
Also, he did not know the regulations of the Majûs in regards to their payment of the jizyah (money tribute). This was until 'Abd-ur-Rahmân Ibn 'Awf informed him that the Messenger of Allâh(swt) said:
And when 'Umar arrived at Sargh  and news reached him that there was a plague in Shâm, he first sought counsel from the Muhâjirîn that were with him, then from the Ansâr, then from the Muslims that were present at the Conquest (of Makkah). Everyone of them advised him according to his own personal opinion and not one of them informed him of the Sunnah (in regards to this situation). This was until 'Abd-ur-Rahmân Ibn 'Awf arrived and informed him of the Sunnah of the Messenger of Allâh(swt) concerning plagues and that he(saw) said:
And he and Ibn 'Abbâs were reminded of what should be done when one has doubt in his prayer as to whether or not he has broken his ablution. And the Sunnah for that affair was not conveyed to him, until 'Abd-ur-Rahmân Ibn 'Awf informed him that the Prophet(saw) would
And one time 'Umar was on a journey when a wind began to blow violently, so he began to say:
Abu Hurairah said:
In these instances, 'Umar did not have knowledge of these matters, until those who were not equal to him in terms of knowledge, conveyed it to him. In other instances, the aspect of the Sunnah (which he did not know of) was not conveyed to him and so he ruled or gave a verdict in opposition to it. This is such as when he gave a ruling concerning the blood tribute to be paid for the (loss of one’s) fingers; that they vary according to their uses. In spite of this, Abu Mûsâ and Ibn 'Abbâs, who were much lower than him in knowledge, knew that the Prophet(saw) said:
– meaning the thumb and the little finger. 
This aspect of the Sunnah was also conveyed to Mu'awiyah during his leadership and so he judged according to it, and the Muslims found no other alternative but to follow that. So this hadîth was not a sign of deficiency in 'Umar, for the hadîth did not reach him.
Also, he as well as his son 'Abdullâh Ibn 'Umar and other righteous people used to forbid the muhrim (one in state of Ihrâm) from putting on perfume, before going into the state of Ihrâm and before the Ifâdah to Makkah, after the casting of the pebbles at 'Aqabah. And the hadîth of 'آ'ishah did not reach them in which she said:
Likewise, he used to command the one who was wearing khuffs to wipe over them (during ablution), even if he had sandals on, without restricting a fixed period of time for doing it. Many of the Salaf followed him in that matter and they were not conveyed the ahâdîth in which the time period (for its validity) is mentioned. And these ahâdîth are authentically transmitted by some of the companions who were not of equivalent status to him in knowledge. Furthermore, information on this matter has been reported on the Messenger of Allâh(swt) through numerous authentic narrations. 
The same goes for 'Uthmân for he did not possess knowledge that the woman whose husband passed away was to complete her 'iddah (waiting period) in her (deceased) husband's home. This was until he heard the hadîth of Furai'ah Bint Mâlik, the sister of Abu Sa'îd Al-Khudrî, in regards to her situation when her husband died. And it was that the Prophet(saw) told her:
So 'Uthmân based his opinion on this (hadîth). 
And one time he was given some hunting game, which was caught just for his purpose, as a gift. So he was about to eat it, when 'Alî Ibn Abî Tâlib informed him that:
Similarly, 'Alî said:
Then he mentioned the famous hadîth of the prayer of repentance. [18) ]
Also he, 'Ibn 'Abbâs and others declared the ruling that the woman whose husband dies while she was pregnant, had to complete the longer of the two waiting periods. And the Sunnah of the Messenger of Allâh(swt) concerning this matter in the case of Subai'ah Al-Aslamiyyah did not reach them. For when her husband, Sa'ad Ibn Khaulah, died, the Prophet(saw) ruled that
Also, he along with Zaid, Ibn 'Umar and others ruled that a women that is married to a man while being in another land, if her husband dies, that she receives no dowry. In this example, the Sunnah of the Messenger of Allâh(swt) concerning the case of Barwa' Bint Wâshiq did not reach them. 
Discussion on this topic is extremely vast, such that the reports on the companions of the Messenger of Allâh(swt) concerning it are numerous and many. As for the reports concerning its occurrence with other than them, from those who came after them, then taking account of all of them is impossible, for verily they reach the level of thousands. And it must be kept in mind that these four companions were the most knowledgeable, the most understanding, the most submissive and fearing, as well as the most virtuous from the entire ummah. And those who came after them are much lower than them in this regard, thus, it is more likely that some aspects of the Sunnah would be unknown to them. And this needs no explanation.
So whoever believes that every hadîth that was conveyed to one of the Imâms or to one specific Imâm, is authentic, then he is mistaken. And that is a vile and detestable mistake.
And no one should say: "Indeed (all) the hadîth were transcribed in books and collected, so how could it be hidden from anyone?" This statement is far from correct for indeed these transcription of ahâdîth, which are well-known in the books of Sunnah, were only collected after the death and disappearance of these Imâms whom are followed by the people, may Allâh have mercy on them. In spite of this, it is not permissible for it to be claimed that all of the hadîth of the Messenger of Allâh(swt) are limited to certain specified books.
And even if one were to limit all of the hadîth of the Messenger of Allâh(swt) to such, then still, everything that was in these books was not known to a scholar, nor can this be attained by anyone. Rather there would be with one man, many records of transcribed ahâdîth and he would not be able to grasp all that was in them. Rather, the ones who existed before these books of transcribed ahâdîth were collected, were much more knowledgeable of the Sunnah than those who came after them. This is because a lot of what was conveyed to them and considered authentic in their opinion, did not reach us except with a narrator that was unknown or with a cut chain of narration or it was not conveyed to us in its complete form. Thus the place where they would transcribe the ahâdîth was in their hearts – and there would be vast amounts of these recorded transcriptions held within them. This is one matter from the knowledge of legal rulings, of which there is no doubt.
And no one should say: "Anyone that doesn’t know all of the ahâdîth cannot be a mujtahid." This is because if it were a condition for a mujtahid to have knowledge of everything that the Prophet(saws) said and did concerning matters related to rulings, then this ummah would never see this type of mujtahid. Indeed, the extent of a scholar is only: that he knows a majority or a great part of the ahâdîth, such that there does not remain anything unknown to him except for a few cases from the general whole. It is only then that these few cases from the general whole (that are unknown to him), can oppose what has reached and been conveyed to him from knowledge.
2. Translators note: This term refers to the deducing of a verdict based on the lack of there being any evidence to forbid it. This applies to only matters outside the field of belief and worship, such as business, family and social affairs. And Allâh knows best.
3. Al-Albânî said in Irwâ-ul-Ghalîl (no. 1680): “It is reported by Abu Dawûd and At-Tirmidhî from the hadîth of Qabîsah Ibn Dhu’aib in mursal form. It has different paths of narration that are all mursal, including the hadîth of ‘Imrân Ibn Husain.”
5.Reported by Al-Bukhârî in his Sahîh from Abu Sa’îd Al-Khudrî. See Fath-ul-Bârî (11/43).
6. Translators Note: A group of people that saw to it that the blood money was properly dispersed and Allâh knows best.
7.Reported by Ahmad, Abu Dawûd and At-Tirmidhî, who said it was a hasan sahîh hadîth. See Irwâ-ul-Ghalîl (no. 2649)
8.Ash-Shâfi’î reported it in his Musnad in mursal form and it has other paths of narration in mursal form with this same wording. Ahmad, Al-Bukhârî, Abu Dawûd and At-Tirmidhî reported from ‘Umar(ra) that: “He would not collect the jizyah (money tribute) from the Majûs until ‘Abd-ur-Rahmân Ibn ‘Awf(ra)bore witness that the Messenger of Allâh took it from the Majûs of Hajr.”
9.This is an area of land located in the last parts of Shâm and the first parts of Hijâz, between Al-Mughîtha and Tabûk.
10.Reported by Ahmad, Al-Bukhârî and Muslim from ‘Abd-ur-Rahmân Ibn ‘Awf
11. Al-Albânî said: “Ahmad, Muslim, Abu Dawûd and At-Tirmidhî reported it, however it is from Abu Sa’îd Al-Khudrî. As for the report of ‘Abd-ur-Rahmân Ibn ‘Awf, then Ahmad, At-Tirmidhî and Ibn Mâjah reported it with the wording:
‘If one of you should get doubt in his prayer, such that he does not know if he has prayed one or two (raka’ahs), then he should make it one...’
And there is no mention of ‘casting away the doubts’ and ‘building upon what one is certain of’ as has been stated by the author, may Allâh have mercy on him.”
12. Al-Albânî said:
“It is that which Muslim has reported in his Sahîh from ‘Aa’ishah(ra)that she said: ‘When the wind would blow violently, the Prophet (saw) would say:
‘O Allâh, I ask You for the good of it, the good of what is found in it and the good of what You send with it. And I seek refuge in You from the evil of it, the evil of what is found in it and the evil of what You send with it.’
And it is what Abu Dawûd and Ibn Mâjah have reported on Abu Hurairah(ra) that he heard the Messenger of Allâh say:
‘The wind is form the spirit (rûh) of Allâh. It comes with mercy and it comes with punishment. So if you see it, then do not revile it. But (instead) ask Allâh for the good of it and seek refuge in Allâh from the evil of it.’
And it is a hasan sahîh hadîth as has been stated by Al-Hâfidh Ibn Hajr (ra).”
13. Reported by Al-Bukhari, Abu Dawûd, Ibn Mâjah and An-Nasâ’î from Ibn ‘Abbâs (ra)
14. An agreed upon hadîth (i.e. reported by Al-Bukhârî and Muslim).
15. Al-Albânî said:
“Ahmad and Muslim reported it from the hadîth of ‘Alî. And Ahmad, Abu Dawûd and At-Tirmidhî reported it form the hadîth of Khuzaimah Ibn Thâbit. And it has been reported from the hadîth of Safwân Ibn ‘Assâl(ra)by An-Nasâ’î, At-Tirmidhî and Ibn Khuzaimah, and the last two authenticated it. Ad-Dâraqutnî reported it and authenticated it, as did Ibn Khuzaimah from the hadîth of Abu Bakrah. These ahâdîth provide evidence for the time period of wiping over the footwear, which is one day and one night for the resident and three days and three nights for the traveler.
At-Tirmidhî said: ‘This is the opinion of the scholars from the Companions of the Prophet (saw) as well as those that came after them from the Fuqahâ.’”
16.The compilers of the Sunans reported this and At-Tirmidhî, Ibn Hibbân, Al-Hâkim and others authenticated it. It is from the hadîth of Furai’ah Bint Mâlik. See Al-Musnad (6/370).
17. Reported by Ahmad in Al-Musnad (1/100 & 104)
18. Al-Albânî said:
19. Reported by Al-Bukhârî, Muslim, Abu Dawûd and Ibn Mâjah with close and similar wordings.
20. Reported by Imâm Ahmad and the compilers of the Sunans. At-Tirmidhî authenticated it. And her husband was Hilâl Ibn Murrah Al-Ashja’î.