(47) باب ذِكْرِ الْخَوَارِجِ وَصِفَاتِهِمْ
Abu Sa'id al-Khudri reported from the Messenger of Allah (ﷺ) that a group (Khawarij) 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.
Reference: Sahih Muslim 1065 e
In-book reference: Book 12, Hadith 198
USC-MSA web (English) reference(deprecated numbering scheme): Book 5, Hadith 2327