Sayyidah Aishah (may Allah be pleased with her) said that the Prophet (peace and blessings be upon him) said: "It is not lawful for anyone to kill a Muslim who recites the kalimah (testifying that there is no god but Allah and that I am the Messenger of Allah), except for one of three reasons: first, a married adulterer; second, by way of retaliation (qisas); and third, a man who goes out waging war against Allah and His Messenger: he is either to be killed, or crucified, or exiled from the land."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السرقة / 17312