Musnad Abu Ya'la al-Mawsili 110:207

Narrated to us Ishaq, narrated to us Abd al-Samad ibn Abd al-Warith, narrated to us Hisham, from Qatadah, from Anas: That Ali was brought people from the Zutt whom they found worshipping an idol, so he burned them. This reached Ibn Abbas, so he said: "The Messenger of Allah, may Allah bless him and grant him peace, only said: 'Whoever changes his religion, then kill him.'"