Sahih Muslim
The Book of Destiny (Kitab-ul-Qadr)
Book Number: 033 / Hadith Number: 6434
Ubayy b. Ka'b reported that AUah'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.