Sahih Muslim Book 5, Hadith Number 2326.

Chapter : Exhortation to kill the Khwarij.

Abu Sa’id al-Khudri reported Allah’s Messenger (may peace be upon him) as saying: There would be two groups in my Ummah, and there would emerge another group (seceding itself from both of them), and the party nearer to the truth among the two would kill them (the group of the Khwarij).