Musnad Ahmad 33:688

Hashim narrated to us, Abu Ja'far narrated to us, from al-Rabi' ibn Anas, from al-Hasan, from Jabir ibn Abdullah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Every newborn is born upon the *fitra* (natural disposition), until his tongue expresses it for him. So when his tongue expresses it for him, he is either grateful or ungrateful."