Musnad Ahmad Hadith 21109

It is narrated from Ibn Abbas (may Allah be pleased with them both) that once he and Har bin Qais Fazari had a difference of opinion regarding the companion of Musa (peace be upon him) to whom he had requested permission from Allah to travel and meet. Ibn Abbas (may Allah be pleased with them both) was of the opinion that he was Khidr (peace be upon him). Meanwhile, Ubayy bin Ka'b (may Allah be pleased with him) happened to pass by. Ibn Abbas (may Allah be pleased with them both) called out to him and said, "My companion and I have differed regarding the identity of the companion of Musa (peace be upon him) whom he requested to travel and meet. Have you heard anything from the Prophet (ﷺ) regarding this?" He replied, "Yes! I heard the Prophet (ﷺ) say: Once Musa (peace be upon him) was addressing a gathering of Bani Israel when a man stood up and asked him, 'Is there anyone more knowledgeable than you?' Musa (peace be upon him) replied, 'No.' Upon this, Allah revealed to him that 'Our servant Khidr is more knowledgeable than you.' Musa (peace be upon him) asked how he could meet him, so Allah made a fish a sign for him and said, 'When you do not find the fish, return, for there you will meet him.' Musa (peace be upon him) set out on the journey, and at one place he stopped and said to his servant, 'Bring our breakfast, for we have faced much hardship in this journey.' There Musa (peace be upon him) found the fish missing, so both of them retraced their footsteps and returned, and then that incident occurred which Allah has mentioned in His Book."