The Prophet ﷺ tells it, in a hadith reported by Ubayy ibn Ka'b (may Allah be pleased with him).
Mûsâ stood addressing the Children of Israel. He was asked: “Who is the most learned of people?”
He said: “I am.” Allah reproached him, because he did not admit that knowledge comes from Allah.
Then Allah revealed to him: at the junction of the two seas is a servant of Mine who knows more than you.
He is one of the greatest prophets, and still he receives this lesson: knowledge belongs to Allah first.
“How can I meet him?” He was told: carry a fish in a basket. Where you lose the fish, that servant is there.
And when Mûsâ said to his young servant: “I will not stop until I reach the junction of the two seas, or go on for ages.”
The verse does not name the young servant. The hadith does: Yûsha' ibn Nûn.
Ibn Kathîr adds that this Yûsha' is the one who led the Children of Israel after Mûsâ.
When they reached the junction between them, they forgot their fish, and it took its way into the sea, slipping away.
In the hadith, they had slept by a rock. The fish slipped from the basket into the sea, as through a tunnel.
They walked on. Then Mûsâ said: “Bring us our meal. This journey has worn us out.”
The hadith notes that he felt no fatigue until he had passed the place he had been told of.
He said: “Do you remember when we rested at the rock? I forgot the fish there.”
Mûsâ said: “That is what we were seeking.” So they went back, retracing their steps.
What looked like a loss was the sign: the lost fish showed the exact place.
There they found a servant from among Our servants, to whom We had given mercy from Us,
and whom We had taught knowledge from Us.
The Qur'an calls him only “a servant”. The hadith gives his name: al-Khidr.
Was he a prophet? Scholars differ. Ibn Kathîr thinks so, because of his words “not of my own accord” (18:82).
Mûsâ said to him: “May I follow you, so that you teach me some of the right guidance you were taught?”
A prophet asking to be a student. He does not demand, he asks: “May I follow you?”
He said: “You will never be able to have patience with me.
And how can you be patient over what your knowledge does not encompass?”
In the hadith he adds: I have knowledge from Allah that you do not have, and you have knowledge that I do not have.
Mûsâ promised to be patient, if Allah wills. Al-Khidr's condition: ask nothing until he speaks of it.
Mûsâ (peace be upon him) was addressing his people when someone asked: who is the most learned?
His answer sent him on a journey to the place where two seas meet.
Mûsâ crossed land and sea to learn from someone who knew what he did not.
Tonight, think of one person who knows something you don't, and ask them to teach you.