Musnad Ahmad 30:594

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Ibn al-Musayyib, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Every newborn is born upon the *fitrah* (natural disposition), then his parents make him a Jew, or a Christian, or a Magian, just as an animal produces offspring—do you perceive any of them mutilated?" Then Abu Hurayrah says: "And recite if you wish: {The natural disposition of Allah upon which He has created mankind. No change let there be in the creation of Allah} [Ar-Rum: 30]."