Musnad Ahmad 26:74

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Abdullah ibn Murrah, from Masruq, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The blood of a Muslim man who testifies that there is no god but Allah and that I am the Messenger of Allah is not lawful, except for one of three: the previously married adulterer, a life for a life, and the one who abandons his religion and separates from the community."