Sahih Muslim Book 33, Hadith Number 6426.

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.

Abu Huraira reported Allah’s Messenger (may peace be upon him) as saying: No babe is born but upon Fitra. It is his parents who make him a Jew or a Christian or a Polytheist. A person said: Allah’s Messenger, what is your opinion if they were to die before that (before reaching the age of adolescence when they can distinguish between right and wrong)? He said: It is Allah alone Who knows what they would be doing.