The Messenger of Allah ﷺ said, "Every child is born upon the natural disposition, then his parents make him a Jew or a Christian, just as camels produce offspring with intact bodies. Do you find any of them mutilated?"They asked, "Messenger of Allah, what do you say about those who die while still young?"He replied, "Allah knows best what they would have done."
حدثنا القعنبي عن مالك عن أبي الزناد عن الأعرج
قال رسول الله ﷺ كل مولود يولد على الفطرة فأبواه يهودانه وينصرانه كما تناتج الإبل من بهيمة جمعاء هل تحس من جدعاء قالوا يا رسول الله أفرأيت من يموت وهو صغير قال الله أعلم بما كانوا عاملين
It was narrated by Muslim, Book of Qadar, Chapter on the meaning of "every child is born upon fitrah," etc., hadith no. 2659, from the hadith of Abu Zinad with this wording. It is also in the Muwatta (Yahya): 1/241, and from this route it was narrated by Ibn Hibban (Ihsan): 133.