Musnad Ahmad 30:1060

The Messenger of Allah, may Allah bless him and grant him peace, said: "There is no newborn who is born but upon this *fitra* (natural disposition), then his parents make him a Jew or make him a Christian, just as you produce camels; do you find among them any that are mutilated until you yourselves mutilate them?" They said: "O Messenger of Allah, what about one who dies while he is young?" He said: "Allah is most knowing of what they would have been doing."