Sahih Muslim Book 33, Hadith Number 6427.

Chapter : Everyone is born according to his true nature and the command pertaining to the demise of the children of the infidels and of the children of the Muslims.

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 narrated 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.”