"Any of the infidels who said, ‘There is no God but God, and Muhammad is the apostle of God’, he became a Muslim obligated to Islamic laws. If he rejected that later on, he would be subject to death. But if he was one of the people of the Book (namely, from the Jews or Christians), in order to become a Muslim, he must say, ‘I have embraced Islam.’ Then he becomes a Muslim obligated to the Islamic laws. If he rejected them, he would be killed." (ibn Hazm, "The Sweetened" (Al-Muhalla, vol. 4, p. 316)