Bukhari, volume 9, #17 "Narrated Abdullah: Allah's Messenger said, "The blood of a Muslim who confesses that none has the right to be worshipped but Allah and that I am His Messenger, cannot be shed except in three cases: in Qisas (equality in punishment) for murder, a married person who commits illegal sexual intercourse and the one who reverts from Islam (Apostate) and leaves the Muslims."