Musnad Ahmad 4039

It was narrated that `Abdullah (رضي الله عنه) said: The Messenger of Allah (ﷺ) said: "The blood of a Muslim man who bears witness that there is no god but Allah and that I am the Messenger of Allah is not permissible (to be shed) except in one of three cases: a married adulterer, a soul for a soul, and one who leaves his religion and separates from the jama'ah (the main body of Muslims)."