Allah’s Messenger صلى الله عليه وسلم 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 for murder, a married person who commits illegal sexual intercourse and the one who reverts from Islam (apostate) and leaves the Muslims.”

Narrated Abdullah. Sahih Bukhari. Volume 9, Book 83, Number 17