Sayyiduna Abd al-Rahman bin Thawban (may Allah be pleased with him) says: The Prophet (peace and blessings be upon him) said in his sermon: "Indeed, in this town (by which the Prophet (peace and blessings be upon him) meant Madinah) it is not fitting that there be two qiblahs. So any Christian who becomes Muslim and then returns to Christianity, strike off his neck."
Al-Tabarani recorded it. Its chain contains narrators whom I do not know.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الحدود والديات / 10575