Musannaf Ibn Abi Shayba 19:934

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, from Ayyub, from ‘Ikrimah, from Ibn ‘Abbas, that it reached him that ‘Ali had seized some heretics (zanadiqa) and burned them. He said: So he said, "As for me, had I been [in his place], I would not have punished them with the punishment of Allah. And had I been [in his place], I would have killed them, because of the saying of the Prophet, peace and blessings of Allah be upon him: 'Whoever changes his religion, kill him.'"