Sahih al-Bukhari 74

Narrated Ibn `Abbas:That he differed with Hur bin Qais bin Hisn Al-Fazari regarding the companion of (the Prophet) Moses. Ibn `Abbas said that he was Al Khadir. Meanwhile, Ubai bin Ka`b passed by them and Ibn `Abbas called him, saying "My friend (Hur) and I have differed regarding Moses' companion, whom Moses asked the way to meet. Have you heard the Prophet (ﷺ) mentioning something about him? He said, "Yes. I heard Allah's Messenger (ﷺ) saying, "While Moses was sitting in the company of some Israelites, a man came and asked him. "Do you know anyone who is more learned than you? Moses replied: "No." So Allah sent the Divine Inspiration to Moses: 'Yes, Our slave Khadir (is more learned than you.)' Moses asked (Allah) how to meet him (Khadir). So Allah made the fish as a sign for him and he was told that when the fish was lost, he should return (to the place where he had lost it) and there he would meet him (Al-Khadir). So Moses went on looking for the sign of the fish in the sea. The servant-boy of Moses said to him: Do you remember when we betook ourselves to the rock, I indeed forgot the fish, none but Satan made me forget to remember it. On that Moses said: 'That is what we have been seeking' (18.64). So they went back retracing their footsteps, and found Khadir. (And) what happened further to them is narrated in the Holy Qur'an by Allah. (18.54 up to)