Sahih Muslim Book 5, Hadith Number 2324.

Chapter : Exhortation to kill the Khwarij.

Abu Sa’id al-Khudri said that the Apostle of Allah (may peace be upon him) made a mention of a sect that would be among his Ummah which would emerge out of the dissension of the people. Their distinctive mark would be shaven heads. They would be the worst creatures or the worst of the creatures. The group who would be nearer to the truth out of the two would kill them. The Apostle of Allah (may peace be upon him) gave an example (to give their description) or he said: A man throws an arrow at the prey (or he said at the target), and sees at its iron head, but finds no sign (of blood there), or he sees at the lowest end, but would not see or find any sign (of blood there). He would then see into the grip but would not find (anything) sticking to it. Abu Sa’id then said: People of Iraq, it is you who have killed them.