Sahih Muslim
The Book of Zakat (Kitab Al-Zakat)
Book Number: 005 / Hadith Number: 2327
Abu Sa'id al-Khudri reported from the Apostle of Allah (may peace be upon him) that a group (Khwarij) would emerge from the different parties (the party of Hadrat 'Ali and the party of Amir Mu'awiya), the group nearer the truth between the two would kill them.