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 soul for a soul, a married adulterer, and one who leaves his religion and separates from the jama'ah (the main body of Muslims)."