The Messenger of Allah ﷺ said, "The boy whom Khidr killed was created a disbeliever by nature, and had he lived, he would have overwhelmed his parents with rebellion and disbelief."
قال رسول الله ﷺ: إن الغلام الذي قتله الخضر طبع كافرا، ولو عاش لأرهق أبويه طغيانا وكفرا.
Muslim narrated it in the Chapter on Qadar (2661) from Abdullah bin Maslama, who said, Mutamir bin Sulayman told us, from his father, from Raqaba bin Masqala, from Abu Ishaq, from Saeed bin Jubayr, from Ibn Abbas, from Ubayy bin Kaab, and he mentioned it.His saying, "believers," means upon the religion of Ibrahim . And his saying, "stamped as a disbeliever," means when he grows up and lives.