It is narrated from Ibn Abbas (may Allah be pleased with them both) with reference to Ubayy bin Ka'b (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: The boy whom Khidr (peace be upon him) killed was naturally a disbeliever (if he had lived, he would have led his parents close to rebellion and disbelief).
Hadith Referenceمسند احمد / مسند الأنصار رضي الله عنهم / 21122
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد حسن