Musannaf Ibn Abi Shayba 28:223

'Abd al-Rahim ibn Sulayman narrated to us, from Shayban al-Nahwi, from Yahya ibn Abi Kathir, from Muhammad ibn 'Abd al-Rahman ibn Thawban, that the Messenger of Allah (may Allah bless him and grant him peace) said in the last sermon he delivered: "Indeed, this village—meaning Medina—is not suitable for two religions. So, whichever Christian accepts Islam and then reverts to Christianity, strike his neck."