Musnad Ahmad 3:111

Sulayman ibn Harb narrated to us, Hammad ibn Zayd narrated to us, from Yahya ibn Sa'id, from Abu Umamah ibn Sahl ibn Hunayf, who said: We were with Uthman while he was besieged in the house. He said: "Why would they kill me? I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'The blood of a Muslim man is not lawful except for one of three: a man who disbelieved after his Islam, or committed adultery after his chastity, or killed a soul, so he is killed for it.'"