It was narrated that Abu Umamah bin Sahl bin Hunaif said. We were with Uthman when he was besieged in the house. He said:
Why would they kill me? I heard the Messenger of Allah (ﷺ) say: `It is not permissible (to shed) the blood of a Muslim except in three cases: If a man disbelieves after becoming Muslim, or commits zina after being married or kills someone and is executed in retaliation.`
https://sunnah.com/bulugh/9/46