Musnad Abu Ya'la al-Mawsili 110:206

Ishaq ibn Abi Isra'il narrated to us, Sufyan ibn 'Uyaynah and Hammad ibn Zayd narrated to us, from Ayyub, from 'Ikrimah, from Ibn 'Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever changes his religion, kill him."