Sahih Bukhari Volume 004, Book 055, Hadith Number 612.
Narated By Ibn Abbas : That he differed with Al-Hur bin Qais Al-Fazari regarding the companion of 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 and I have differed regarding Moses' companion whom Moses asked the way to meet. Have you heard Allah's Apostle mentioning something about him?" He said, "Yes, I heard Allah's Apostle 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 how to meet him (i.e. Khadir). So, the fish, was made, as a sign for him, and he was told that when the fish was lost, he should return and there he would meet him. So, Moses went on looking for the sign of the fish in the sea. The servant boy of Moses said to him, 'Do you know that when we were sitting by the side of the rock, I forgot the fish, and t was only Satan who made me forget to tell (you) about it.' Moses said, That was what we were seeking after,' and both of them returned, following their footmarks and found Khadir; and what happened further to them, is mentioned in Allah's Book."