Understand that judging by what Allaah revealed in His Book is incumbent upon the Muslim, and not doing so results an many calamities befalling the Ummah.
That there would always continue to be a group from this Ummah clinging to the truth and to the pure Tawheed of Allaah until the day of Judgment.
Allaah - the Most High - said:
“Say: Allaah has power to send punishment upon you from above you, or from under your feet; or to cover you in confusion with party strife; and make you taste the inter-fighting and violence of one another. See how We variously explain the Aayaat (proofs and signs), so that they may understand.” [Sooratul-An’aam 6:65]
Allaah’s Messenger (sallallaahu ’alayhi wa sallam) said, when this verse was revealed: “Say: Allaah has power to send punishment upon you from above you.” So he said: “I seek refuge in Your Face.” “Or from under your feet.” He said: “I seek refuge in Your Face.”
“Or to cover you in confusion with party strife, and make you taste the inter-fighting and violence of one another.” So Allaah’s Messenger (sallallaahu ’alayhi wa sallam) said: “This is lighter or easier.” 
The Prophet (sallallaahu ’alayhi wa sallam) said: “Indeed Allaah gathered up the earth for me so that I saw its eastern and western parts, and indeed the dominion of my Ummah will reach what was gathered up for me from it. And I have been granted the two treasures of gold and silver. And indeed I asked my Lord for my Ummah that it should not be totally destroyed by a prevailing famine, nor to allow on enemy to totally conquer it, except from amongst themselves. So my Lord said: O Muhammad! When I decree a matter, then it is not reversed. I grant to you for your Ummah that it would not be totally destroyed by a prevailing famine, nor will it be totally conquered by on external enemy, even if all the notions were to rally against it However, a group from your Ummah will destroy one another and imprison one another.” 
The Prophet (sallallaahu ’alayhi wa sallam) also said: “I asked my Lord for three things, I was granted two; but prevented from one: I asked my Lord that He should not destroy my Ummah with drought and famine, so He granted it to me. I asked my Lord that He should not destroy my Ummah with drowning, so he granted it to me. And I asked Him not to cause my Ummah to fight amongst themselves, but he refused that to me.” 
And this inter-fighting that has befallen the Muslims has been further emphasized in his (sallallaahu ’alayhi wa sallam) saying: “And when your leaders do not judge by the Book of Allaah, not seeking the good and welfare with Allaah has revealed, then Allaah would cause them to fight one another.” 
The Prophet said, in an addition to the second narration: “Indeed, what I fear for my Ummah are misguided leaders, and that when the sword is drown out against them, it will not be lifted from them until the Day of Judgement. And the Hour will not come, until sections of my Ummah will follow the mushrikeen (those who worship others along with Allaah), and until many sections of my Ummah will worship idols. And there will be in my Ummah thirty liars; all of whom will claim that he is a Prophet. But I am the last and the seal of the Prophets; there is no Prophet after me. And there will not cease to be a group from my Ummah being helped upon the truth, not being harmed by those who oppose them; until the commend of Allaah comes.” 
SOME BENEFITS FROM THE ABOVE AAYAH AND HADEETH:
: It shows the great power of Allaah - the Most High – in sending punishment to the previous nations, from above them, and from beneath their feet, causing them to be annihilated and destroyed. Due to this, Allaah’s Messenger (sallallaahu ’alayhi wa sallam) sought refuge and protection in the Face of his Lord against his Ummah being punished in a similar manner.
: The first hadeeth (narration) affirms that Allaah – the Most High – has a wajh (Face), but in a manner that befits His Greatness and Majesty, there being any tashbeeh (resemblance) between Allaah and between that of His creation; as Allaah said: “There is none like Him; and He is the all-Hearing, the all-Seeing.” [Sooratush-Shooraa 42:11]
: It shows the great power of Allaah - the Most High - in that whenever the Muslims abandon judging by the Sharee’ah (Divinely Prescribed Law) of Allaah; but rather take to manmade laws that oppose the Law of Allaah - which, with deep regret, is happening now - then Allaah causes the Ummah to split-up into sects and parties, and makes them fight one another.
: It Shows the might of Allaah in that He gathered-up the earth for His Messenger (sallallaahu ’alayhi wa sallam), so that he saw its eastern and western parts; and that the dominion of his Ummah would extend over all that which was gathered-up for him, from it.
: It shows the mercy of Allaah to the Muslim Ummah, in His not sending against them a punishment that would totally destroy the entire Ummah; but it would only cause destruction to parts of the Ummah.
: An-Nawawee (d.676H) - rahimahullaah - said: “In this hadeeth is a clear and apparent miracle, all of which has come to pass - so all praise be to Allaah.” 
: It also shows the mercy of Allaah to the Muslim Ummah in that He has not allowed an external enemy to overpower them, such that they have complete control over them. Rather, Allaah has caused some parts of the Ummah to fight against others; and this is far easier than the former case.
: It Shows the fear that Allaah’s Messenger (sallallaahu ’alayhi wa sallam) had for his Ummah with regards to the misguided leaders who would not take hold of the Book and the Sunnah; and how he warned against them.
: That when the sword has been drawn out against this Ummah, then it would not be lifted from it until the Day of Judgement - as is happening at this very time now.
: It proves that shirk would be present in the Ummah. And shirk is: Directing worship, or forms of worship, to other than Allaah, such as making du’aa (supplication) to other that Allaah, or judging by other than what Allaah has legislated and revealed.
: We are informed about the existence of those who would claim that they are Prophets and that they are all liars in this claim; such as Musaylimah al-Kadhdhaab in earlier times, and in more recent times Ghulaam Ahmad Mirza, the Qaadiyaanee (whose followers are called Qaadiyaanees).
: That there would always continue to be a group from this Ummah clinging to the truth and to the pure Tawheed of Allaah until the day of Judgement.
Imaam an-Nawawee - rahimahullaah - said:
“As for this group, then Imaam al-Bukhaaree (d.256H) said: “They are the people of knowledge.” And Imaam Ahmad Ibn Hanbal (d.241H) said: “If they are not Ahlul-Hadeeth (the Scholars of Hadeeth), then I do not know who they can be.” Qaadee ’Iyaad said: “What Imaam Ahmad meant by that was Ahlus-Sunnah wal-Jamaa’ah and whosoever holds the ’aqeedah (belief) of Ahlul-Hadeeth.” I say: It is possible that this group could be scattered amongst the various categories of Believers, from them:- the brave warriors, the Scholars of Hadeeth, the zuhhaad (those who abstain from the world in a praiseworthy manner), those that enjoin the good and forbid the evil, and other categories of people who do good.” 
Ibnul-Mubaarak (d.181H) - rahimahullaah - said: “In my view they are Ashaabul-Hadeeth (the companions of Hadeeth).” 
So the preferred saying is that this Aided and Victorious Group are those that act in accordance with the authentic hadeeth, not giving precedence to the saying of anyone over and above the saying of Allaah and His Messenger (sallallaahu ’alayhi wa sallam).
 Tawjeehul-Muslimeen ilat-Tareeqin-Nasr wat-Tamkeen (p. 49-53)
2] Related by al-Bukhaaree (no. 4628), from Jaabir (radiyallaahu ’anhu)
3] Related by Muslim (no. 2889), from Thawbaan (radiyallaahu ’anhu)
4] Related by Muslim (no. 2216), from Sa’d Ibn Abee Waqqaas (radiyallaahu ’anhu)
5] Hasan: Related by Ibn Majaah (no. 4019), from Ibn ’Umar (radiyallaahu ’anhu) It was
authenticated by al-Albaanee in as-Saheehah (no. 106).
6] Saheeh: Related by Aboo Daawood (no. 3952), from Thawbaan (radiyallaahu ’anhu) It was authenticated by al-Haafidh Ibn Hajar in al-Mataalibul-’Aaliyah (4/264).
7] Sharh Saheeh Muslim (1/234)
8] Sharh Saheeh Muslim (13/67)
9] Related by al-Khateeb al-Baghdaadee in Sharaf Ashaabul-Hadeeth (p .26)