The blood of a Muslim is lawful only in three cases

On the authority of Ibn Mas'ud (may Allah be pleased with him), that the Messenger of Allah (may Allah bless him and grant him peace) said: "The blood of a Muslim is not lawful except in three cases: the married person who commits adultery, a life for a life, and the one who abandons his religion and departs from the community."

Related by al-Bukhari and Muslim.

[al-Bukhari 6878; Muslim 1676]