Abu Umama b. Sahl b. Hunaif told that when ‘Uthman b. ‘Affan was besieged he looked down and said:
I adjure you by God. Are you aware that God’s Messenger said, “It is not lawful to kill a man who is a Muslim except for one of three reasons: fornication after marriage, or unbelief after accepting Islam, or wrongfully killing someone, for which he may be killed”? I swear by God that I have not committed fornication before or after the coming of Islam, or apostatised since I swore allegiance to God's Messenger, or killed anyone whom God has declared inviolate; so for what reason do you want to kill me?
Tirmidhi, Nasa'i and Ibn Majah transmitted it, and Darimi gives the wording of the tradition.