It is narrated from Uthman bin Affan (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The blood of a Muslim man is lawful in only one of three cases: apostasy after faith, adultery after marriage, or unjustly killing a soul."
Hadith Referenceمسند الإمام الشافعي / كتاب القتل والقصاص والديات والقسامة / 1606