Musnad Ahmad 5:419

'Abdullah narrated to us, Muhammad ibn Abi Bakr al-Muqaddami narrated to me, Hammad ibn Yahya al-Abbah narrated to us, Ibn 'Awn narrated to us, from Muhammad, from 'Ubaydah, who said: When 'Ali killed the people of al-Nahrawan, he said: "Search for him." They found him in a pit beneath the slain, so they brought him out. 'Ali turned to his companions and said: "Were it not that you would become arrogant, I would have informed you of what Allah has promised those who kill these people, upon the tongue of Muhammad, may Allah bless him and grant him peace." I said: "Did you hear it from the Messenger of Allah, may Allah bless him and grant him peace?" He said: "Yes, and by the Lord of the Ka'bah."