Mukhtasar Muslim

Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) reports that the Messenger of Allah (peace and blessings be upon him) said: It is not permissible to kill a Muslim who testifies that there is no deity worthy of worship except Allah and that I am His Messenger, except for one of three reasons: 1. He commits adultery after being married; or 2. Life for life (i.e., he kills someone); or 3. He leaves his religion and separates from the community of Muslims.