Musnad Ahmad 30:3111

Waki’ narrated to us, he said: Al-A’mash narrated to us, from Abu Salih, from Abu Hurairah, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no newborn who is born except upon the creed" — and he said once: "Every newborn is born upon the natural disposition (fitrah)" — "then his parents make him a Jew, or make him a Christian, or make him a polytheist." It was said: "O Messenger of Allah, what about those who die before that?" He said: "Allah knows best what they would have been doing."