Musnad Ahmad 30:327

Abu Mu'awiyah narrated to us, from al-A'mash, from Abu Salih, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no newborn who is born but upon this religion (millah), until his tongue expresses it for him; then his parents make him a Jew, or make him a Christian, or make him a polytheist." They said: "O Messenger of Allah, what about those who died before that?" He said: "Allah knows best what they would have been doing."