Sahih Muslim Book 33, Hadith Number 6434.

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.

Ubayy b. Ka’b reported that Allah’s Messenger (may peace be upon him) said: The young man whom Khadir killed was a non-believer by his very nature and had he survived he would have involved his parents in defiance and unbelief.