Abu Dawud Book 33, Hadith Number 4339.

Narated By ‘Aisha, Ummul Mu’minin : The Apostle of Allah (PBUH) Said: The blood of a Muslim man who testifies that there is no god but Allah and that Muhammad is Allah’s Apostle should not lawfully be shed except only for one of three reasons: a man who committed fornication after marriage, in which case he should be stoned; one who goes forth to fight with Allah and His Apostle, in which case he should be killed or crucified or exiled from the land; or one who commits murder for which he is killed.