It is narrated from Sayyiduna Mu'awiyah bin Haydah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever changes his religion, kill him. Allah does not accept the repentance of a servant who disbelieves after his Islam."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17062