And Allah the Exalted said (in Surah At-Tawbah) «فلولا نفر من كل فرقة منهم طائفة ليتفقهوا في الدين ولينذروا قومهم إذا رجعوا إليهم لعلهم يحذرون‏» : "Why does not a group from every section of them go forth so that they may acquire understanding in religion and so that they may warn their people when they return to them, that they may beware of evil." And a single person can also be called a "ta'ifah" (group), as in this verse (from Surah Al-Hujurat) «وإن طائفتان من المؤمنين اقتتلوا‏» : "If two groups among the believers fight each other..." and in this are included those two Muslims who fight each other (so each Muslim is considered a group). And (in the same Surah) Allah the Exalted said «إن جاءكم فاسق بنبإ فتبينوا‏» : "O you who believe! If a wicked person comes to you with news, verify it..." And if a single report (khabar wahid) was not acceptable, then why would the Prophet (peace and blessings be upon him) appoint one person as a ruler and then send another after him, and why would he say that if the first ruler forgets something, the second ruler should set him upon the way of the Sunnah?

Narrated Malik: We came to the Prophet and we were young men nearly of equal ages and we stayed with him for twenty nights. Allah's Apostle was a very kind man and when he realized our longing for our families, he asked us about those whom we had left behind. When we informed him, he said, "Go back to your families and stay with them and teach them (religion) and order them (to do good deeds). The Prophet mentioned things some of which I remembered and some I did not. Then he said, "Pray as you have seen me praying, and when it is the time of prayer, one of you should pronounce the call (Adhan) for the prayer and the eldest of you should lead the prayer. "