Sahih Muslim
The Book of Destiny (Kitab-ul-Qadr)
Book Number: 033 / Hadith Number: 6425
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: No child is born but upon Fitra. He then said. Recite: The nature made by Allah in which He created man, there is no altering of Allah's nature; that is the right religion."