Sahih Muslim 2465
صحيح مسلم#2465SahihAbida narrated from 'Ali that he made a mention of the Khawarij (and in this connection) said that there would be a person among them with a defective hand (or with a short hand) or a fleshy hand. If you were to exercise restraint, I would tell you what Allah has promised to those who would kill them on the order of Muhammad (ﷺ). I (the narrator) said to him:Did you hear it from Muhammad (ﷺ)? He (Hadrat 'Ali) said: Yes, by the Lord of the Ka'bah; Yes, by the Lord of the Ka'bah; yes, by the Lord of the Ka'bah
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: 12:202