Musnad Ahmad 26:695

Waki' 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 people: a life for a life, the married adulterer, and the one who abandons his religion and separates from the community."