Musnad Ahmad 920:1

Al-Walid ibn Muslim and Muhammad ibn Mus'ab al-Qarqasani narrated to us. Al-Walid said: Al-Awza'i narrated to me. And Muhammad said: Al-Awza'i narrated to us, that al-Zuhri narrated to him, from Ubayd Allah ibn Abd Allah, from Ibn Abbas: That he and al-Hurr ibn Qays ibn Hisn al-Fazari disputed regarding the companion of Musa, whom he sought the way to meet. Ibn Abbas said: "He is Khidr." Then Ubayy ibn Ka'b passed by them, so Ibn Abbas called out to him and said: "I and my companion here have disputed regarding the companion of Musa, whom he sought the way to meet. Did you hear the Messenger of Allah, peace and blessings of Allah be upon him, mention his affair?" He said: "Yes, I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'While Musa was among a gathering of the Children of Israel, a man stood up to him and said: "Do you know anyone more learned than you?" He said: "No." Then Allah revealed to him: "Our servant, Khidr." So Musa asked for the way to meet him, and Allah made the fish a sign for him. It was said to him: "When you lose the fish, return, for you will meet him."'" Ibn Mus'ab said in his narration: "So he descended at a place, and Musa said to his servant: 'Bring us our morning meal; indeed, we have suffered much fatigue from this journey of ours' [Al-Kahf: 62]. It was at that point that he lost the fish. 'So they returned, retracing their footsteps' [Al-Kahf: 64]. And Musa began to follow the track of the fish in the sea." He said: "And their affair was as Allah related in His Book."