Musnad Ahmad 25:692

'Affan narrated to us, Hammad bin Zayd narrated to us, Ayyub narrated to us, from 'Ikrimah, that 'Ali, may Allah be pleased with him, was brought some people from among these heretics (Zanadiqa), and they had books with them. He ordered a fire, and it was kindled, then he burned them and their books. 'Ikrimah said: This reached Ibn 'Abbas, so he said: "If it were me, I would not have burned them, because of the prohibition of the Messenger of Allah, peace and blessings of Allah be upon him. And I would have killed them, because of the saying of the Messenger of Allah, peace and blessings of Allah be upon him: 'Whoever changes his religion, kill him.'" And the Messenger of Allah, peace and blessings of Allah be upon him, said: "Do not punish with the punishment of Allah, the Mighty and Majestic."