Sahih Muslim
The Book of Destiny (Kitab-ul-Qadr)
Book Number: 033 / Hadith Number: 6427
It is reported on the authority of Abu Mu'awiya that (the Holy Prophet) said: Every new-born babe is born on the millat (of Islam and he) remains on this until his tongue is enabled to express himself.
This hadith has been narratted on the authority of Abu Mu'awiya through another chain of transmitters (and the words are):" Every child is born but on this Fitra so long as he does not express himself with his tongue."