Sayyiduna Abdullah (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "The blood of a Muslim is not lawful except for one of three reasons: he is killed in retaliation (qisas), or a married adulterer, or an apostate who abandons the religion and the community."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المرتد / 16818