Sayyiduna Abdullah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: "The blood of a man who bears witness that there is none worthy of worship except Allah and that I am the Messenger of Allah is not lawful except in one of three cases: a life for a life, the married adulterer, and the one who abandons his religion and separates from the community (jama'ah)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المرتد / 16861