Musnad Abu Ya'la al-Mawsili 35:1

Shayban ibn Farrukh narrated to us, Abu Hamza al-Attar Ishaq ibn al-Rabi' narrated to us, al-Hasan narrated to us, from al-Aswad ibn Sari' who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Every newborn is born upon the natural disposition (fitrah) until his tongue expresses it for him; then his parents make him a Jew or make him a Christian."