Abu Huraira reported God’s messenger as saying, “Everyone is born a Muslim, but his parents make him a Jew, a Christian, or a Magian; just as a beast is born whole. Do you find some among them [born] maimed?” Then he was saying, “God’s pattern on which He formed mankind. There is no alteration of God’s creation. That is the true religion.” 1
(Bukhari and Muslim.)
1 Quran, xxx, 30. It is not quite clear whether these words were recited by the Prophet or by Abu Huraira.