Musannaf Ibn Abi Shayba 28:615

Ibn ‘Uyaynah narrated to us, from Ayyub, from ‘Ikrimah, from Ibn ‘Abbas, that he mentioned some people whom ‘Ali had burned. He said: "If it were I, I would not have burned them with fire, because of the saying of the Messenger of Allah (ﷺ): 'Do not punish with the punishment of Allah.' And if it were I, I would have killed them, because of the saying of the Messenger of Allah (ﷺ): 'Whoever changes his religion, kill him.'"