Sahih Muslim Book 5, Hadith Number 2325.
Chapter : Exhortation to kill the Khwarij.
Abu Sa’id al-Khudri reported that the Messenger of Allah (may peace be upon him) said: A group would secede itself (from the Ummah) when there would be dissension among the Muslims. Out of the two groups who would be nearer the truth would kill them.