Muhammad bin 'Abdullah محمد بن عبد الله بن بزيع · The Book of Fighting [The Prohibition of Bloodshed]

"I heard the Messenger of Allah say: 'It is not permissible to shed the blood of a Muslim except in three cases: A man who commits adultery after having married; or one who kills another person, who is to be killed; or who reverts to Kufr after having accepted Islam, who is to be killed.'"

"Abu Bakr got very angry with a man, so much so that his color changed. I said: 'O Khalifah of the Messenger of Allah, if you tell me to, I will strike his neck (kill him).' It was as if cold water had been poured on him and he became calm. He said: 'May your mother be bereft of you, Abu Barzah! That is not for anyone after the Messenger of Allah .'"