Musnad Ahmad 1153:1776

Waki’ narrated to us, he said: Sufyan narrated to us, from Abu Ishaq, from ‘Amr ibn Ghalib, from ‘Aisha, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The blood of a Muslim man is not lawful except for a man who killed, so he is killed; or a man who committed adultery after having been married (and having had sexual intercourse); or a man who apostatized after his Islam."