(47) باب ذِكْرِ الْخَوَارِجِ وَصِفَاتِهِمْ
Abu Sa'id al-Khudri reported Allah's Messenger (ﷺ) 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 Khawarij).
Reference: Sahih Muslim 1065 c
In-book reference: Book 12, Hadith 196
USC-MSA web (English) reference(deprecated numbering scheme): Book 5, Hadith 2326