From Sahih Muslim (Hadith 2465):
Abida 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
Reference: Muslim 2465