Musnad Ahmad 5:65

Isma'il narrated to us, Ayyub narrated to us, from Muhammad, from 'Ubaydah, from 'Ali, who said: "He mentioned the Khawarij and said: 'Among them is the one with the malformed hand' — or the one with the stunted hand, or the one with the fleshy-defective hand. 'Were it not that you would become arrogant, I would tell you what Allah has promised those who kill them, upon the tongue of Muhammad.' I said: 'Did you hear it from Muhammad?' He 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!'"