Musnad Ahmad 30:2194

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Sulayman, from Dhakwan, from Abu Hurayrah, from the Prophet, may Allah bless him and grant him peace, that he said: "Every newborn is born upon the Fitrah (natural disposition), then his parents make him a Jew, or a Christian, or a polytheist."