Musannaf Ibn Abi Shayba 19:930

Abu Bakr narrated to us, he said: Abu Bakr ibn Ayyash narrated to us, from Abu Husayn, from Suwayd ibn Ghafala, that Ali burned some heretics (zanadiqa) in the marketplace. When he threw fire upon them, he said: "Allah and His Messenger have spoken the truth." Then he turned away, and I followed him. Suwayd said: I said, "Yes, O Commander of the Faithful, I heard you saying something." He said: "O Suwayd, I am with ignorant people. If you hear me say, 'The Messenger of Allah said,' then it is the truth."