[Sahih Bukhari : Volume 1, Book 3, Number 124]
Narrated by Ubai Ka’b: The Prophet Muhammad (sal-allahu-alleihi-wasallam) said, “Once the Prophet Musa alaihi salaam stood up and addressed Bani Israel. He was asked ‘Who is the most learned man amongst the people.’ He said ‘ I am the most learned.’
Allah admonished as he did not attribute absolute knowledge to Him (Allah). So Allah inspired to him ‘At the junction of the two seas there is a slave amongst my slaves who is more learned than you.’ Musa alaihi salaam said , ‘O my Lord! How can I meet him?’ Allah said: ‘Take a fish and proceed and you will find him at the place where you will lose the fish.’ So Musa alaihi salaam set out along with his servant boy, Yusha bin Nun.”
Further Explanation is defined in Surah Al-Kahf from Ayahs 60 to 82:
[18. Surah Al-Kahf : Ayah 60-82]
And when Musa alaihi salaam said to his servant: I will not cease until I reach the junction of the two seas or I will go on for years.
So when they had reached the junction of the two (rivers) they forgot their fish, and it took its way into the sea, going away.
But when they had gone farther, he said to his servant: Bring to us our morning meal, certainly we have met with fatigue from this our journey.
He (Servant) said: Did you see when we took refuge on the rock then I forgot the fish, and nothing made me forget to speak of it but the Shaitan, and it took its way into the river; what a wonder!
He (Musa alaihi salaam) said: This is what we sought for; so they returned retracing their footsteps.
Then they found one from among Our servants whom We had granted mercy from Us and whom We had taught knowledge from Ourselves.
Musa alaihi salaam said to him: Shall I follow you on condition that you should teach me right knowledge of what you have been taught?
He (Khizar alaihi salaam) said: Surely you cannot have patience with me.
And how can you have patience in that of which you have not got a comprehensive knowledge?
He (Musa alaihi salaam) said: If Allah pleases, you will find me patient and I shall not disobey you in any matter.
He (Khizar alaihi salaam) said: If you would follow me, then do not question me about any thing until I myself speak to you about it.
So they went (their way) until when they embarked in the boat he made a hole in it. (Musa alaihi salaam) said: Have you made a hole in it to drown its inmates? Certainly you have done a grievous thing.
He (Khizar alaihi salaam) said: Did I not say that you will not be able to have patience with me?
He (Musa alaihi salaam) said: Blame me not for what I forgot, and do not constrain me to a difficult thing in my affair.
So they went on until, when they met a boy, He (Khizar alaihi salaam) slew him. (Musa alaihi salaam) said: Have you slain an innocent person otherwise than for manslaughter? Certainly you have done an evil thing.
He (Khizar alaihi salaam) said: Did I not say to you that you will not be able to have patience with me?
He (Musa alaihi salaam) said: If I ask you about anything after this, keep me not in your company; indeed you shall have (then) found an excuse in my case.
So they went on until when they came to the people of a town, they asked them for food, but they refused to entertain them as guests. Then they found in it a wall which was on the point of falling, so He (Khizar alaihi salaam) put it into a right state. (Musa alaihi salaam) said: If you had pleased, you might certainly have taken a recompense for it.
He (Khizar alaihi salaam) said: This shall be separation between me and you; now I will inform you of the significance of that with which you could not have patience.
As for the boat, it belonged to (some) poor men who worked on the river and I wished that I should damage it, and there was behind them a king who seized every boat by force.
And as for the boy, his parents were believers and we feared lest he should make disobedience and ingratitude to come upon them:
So we desired that their Lord might give them in his place one better than him in purity and nearer to having compassion.
And as for the wall, it belonged to two orphan boys in the city, and there was beneath it a treasure belonging to them, and their father was a righteous man; so your Lord desired that they should attain their maturity and take out their treasure, a mercy from your Lord, and I did not do it of my own accord. This is the significance of that with which you could not have patience.
The Prophet Muhammad (sal-allahu-alleihi-wasallam) said ” May Allah be Merciful to Musa alaihi salaam! Would that he could have been more patient so that we could learn more about their story.”