Musnad Ahmad 3:70

'Affan narrated to us, Hammad bin Zayd narrated to us, Yahya bin Sa'id narrated to us, from Abu Umamah bin Sahl, who said: I was with 'Uthman in the house while he was besieged. He said: We used to enter a place, and when we entered it, we could hear the speech of those who were on the pavement. He said: 'Uthman entered one day for a need, then he came out to us with his color changed. He said: "They are threatening me with death just now." He said: We said: "Allah will suffice you against them, O Commander of the Faithful." He said: He then said: "And for what would they kill me? For 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 without [the killing of] a soul.' By Allah, I have never committed adultery in the pre-Islamic era nor in Islam, nor have I desired a substitute for my religion since Allah, the Almighty and Majestic, guided me, nor have I killed a soul. So for what would they kill me?"