سنن ابن ماجه

It was narrated from 'Abdullah, who is Ibn Mas`ud, that the Messenger of Allah (ﷺ) said: “It is not lawful to shed the blood of a Muslim who bears witness that none has the right to be worshiped but Allah (SWT), and that I am the Messenger of Allah (ﷺ), except in one of three cases: a soul for a soul; a married person who commits adultery, and one who leaves his religion and splits from the Jama`ah.”