On the authority of Ibn Mas’ud, radiyallahu anhu, who said: The Messenger of Allah, sallallahu ‘alayhi wasallam, said:
\"The blood of a man who is a Muslim is not lawful (i.e. cannot be lawfully shed), save if he belongs to one of three (classes): a married man who is an adulterer; life for a life (i.e. for murder); one who is a deserter of his religion, abandoning the community.” [Al-Bukhari & Muslim]