Musnad Ahmad 5:657

Yazid narrated to us, Hisham informed us, from Muhammad, from Ubaydah, who said: Ali said to the people of the river (the Khawarij): "Among them is a man with a deformed hand—or a withered hand, or a truncated hand. Were it not that you would become arrogant, I would have informed you of what Allah has decreed, upon the tongue of His Prophet (peace and blessings of Allah be upon him), for those who kill them." Ubaydah said: So I said to Ali, "Did you hear it?" He said, "Yes, by the Lord of the Kaaba," swearing by it three times.