Musnad Ahmad 30:1981

Husayn narrated to us, Ibn Abi Dhi’b narrated to us, from al-Zuhri, from Abu Salamah, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Every newborn is born upon the *fitrah* (natural disposition), then his parents make him a Jew, or a Christian, or a Magian, just like the beast; does the beast produce anything but that which is whole (without mutilation)?"