Musannaf Ibn Abi Shaybah 34945 — What they said on a man who accepts

Muhammad bin Abd al-Rahman bin Thawban says that in the last sermon the Messenger of Allah (peace and blessings be upon him) delivered, he (peace and blessings be upon him) said: "Indeed, in this town, meaning Madinah, two religions cannot remain. So any Christian who accepts Islam and then becomes a Christian again, strike his neck."