Musnad Ahmad 920:23

Abdullah narrated to us, Muhammad ibn Abbad al-Makki narrated to us, Abdullah ibn Maymun al-Qaddah narrated to us, Ja'far ibn Muhammad al-Sadiq narrated to us, from Ibn Shihab, from Ubaydullah ibn Abdullah, from Ibn Abbas, who said: A man from Banu Fazarah disputed with me regarding the man whom Musa followed. I said: "He is al-Khidr." The Fazari said: "He is another man." Then Ubayy ibn Ka'b passed by us. Ibn Abbas said: "So I called him and asked him: 'Did you hear the Messenger of Allah, peace and blessings of Allah be upon him, mention the one whom Musa followed?'" He said: "Yes, I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'While Musa was sitting among a gathering of the Children of Israel, a man said to him: "Is there anyone more knowledgeable of Allah than you?" He said: "I do not think so." Then Allah revealed to him: "Yes, My servant al-Khidr." So he asked for the way to him, and Allah made the fish a sign for him if he lost it, and his affair was as Allah has related.'"