Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: The blood (killing) of a Muslim who testifies that there is no deity except Allah and that I am the Messenger of Allah is not lawful, except in three cases: a life for a life, a married adulterer, and one who leaves his religion and separates from the (Muslim) community.