Sunan An-Nasa'i

It was narrated that 'Uthman bin 'Affan said:""I heard the Messenger of Allah [SAW] 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