Sahih Muslim 2459
صحيح مسلم#2459SahihAbu 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)
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 12:196