Musnad Ahmad 3:39

Sulayman ibn Harb and ‘Affan narrated to us—the meaning is the same—they said: Hammad ibn Zayd narrated to us, Yahya ibn Sa‘id narrated to us, from Abu Umamah ibn Sahl, who said: We were with ‘Uthman while he was besieged in the house. He entered a place where, when he entered it, his speech could be heard from the pavement. He entered that place and then came out to us and said: "They are threatening me with death just now." We said: "Allah will suffice you against them, O Commander of the Faithful." He said: "And for what 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.' By Allah, I have not desired a substitute for my religion since Allah guided me, nor have I committed adultery in the Age of Ignorance or in Islam ever, nor have I killed a soul. So for what would they kill me?"