"The blood of a Muslim is not to be shed except for three reasons: a married man who committed adultery, a man who kills another Muslim intentionally for which he must be killed (in Qisas or retaliation), and a man who abandons Islam and fights against Allah and His Messenger, in which case he should be either killed, crucified, or exiled." Related by Abu Dawud and An-Nasa'i. Al-Hakim graded it as Sahih.