Musannaf Ibn Abi Shayba 28:624

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 them into the fire, he said, "Allah and His Messenger have spoken the truth." Then he turned away, and I followed him. He turned toward me and said, "Suwayd?" I said, "Yes." I said, "O Commander of the Faithful, I heard you saying something." He said, "O Suwayd, I am among ignorant people. So if you hear me say, 'The Messenger of Allah (may Allah bless him and grant him peace) said,' then it is the truth."