So when they had passed beyond it, [Moses] said to his boy, "Bring us our morning meal. We have certainly suffered in this, our journey, [much] fatigue."
The reason for Musa's conversation with the boy-servant, Yusha` bin Nun, was that he had been told about one of the servants of Allah at the junction of the two seas, who had knowledge which Musa had not been granted, so he wanted to travel to meet him. So he said to that boy-servant of his:
لا أَبْرَحُ
(I will not give up) meaning, I will keep on traveling,
حَتَّى أَبْلُغَ مَجْمَعَ الْبَحْرَيْنِ
(until I reach the junction of the two seas) meaning, the place where the two seas met.
أَوْ أَمْضِىَ حُقُباً
(or a Huqub passes.) meaning, even if I have to travel for a very long time. Ibn Jarir (may Allah have mercy on him) said, "Some of the scholars of the Arabic language said that Huqub means a year in the dialect of the tribe of Qays," then he narrated that `Abdullah bin `Amr said, "Huqub means eighty years." Mujahid said, "Seventy years." `Ali bin Abi Talhah reported that Ibn `Abbas said that it means a lifetime. Qatadah and Ibn Zayd said likewise.
So, when they reached the meeting point of the two seas, they forgot their fish and it made its way into the sea as in a tunnel- 61.
Before we explain the nature of the extraordinary happening there, it is appropriate to first refer to the distinct position bestowed upon Sayyidna Musa 1 in the comity of prophets. This has been mentioned explicitly in the Qur'an, and Sunnah. The special distinction of having conversed with Allah Ta` a1a is his hallmark. With al-Khadir (علیہ السلام) ، the matter is different. To begin with, the very fact of his being a prophet has been debated. Even if his prophet-hood (nubuwwah) is acknowledged, he does not hold the station of a Messenger (Rasul) of Allah ﷺ . Neither is he credited with a Book, nor does he have a distinct community of followers. Therefore, considering all related aspects, Sayyidna Musa (علیہ السلام) has marked precedence over al-Khadir (علیہ السلام) . But, Allah Ta` ala has His own wise ways to help those close to Him perform better. He would not let the least want or short coming go uncorrected and not amended. This process of reform at the highest level would even bring serious displeasure from Allah and it is through an equally serious measure that they are made to repair for it. This whole story reflects the same mode of person-al training. By saying: اَنَا (and: I), he had said: 'I am the foremost in knowledge.' Allah Ta` ala did not like it. So, as a measure of warning, he was given the whereabouts of a servant who had a field of knowledge specially given to him by Allah. This Sayyidna Musa (علیہ السلام) did not have. Though the knowledge of Sayyidna Musa (علیہ السلام) was far higher in rank than that given to him, but the truth of the matter was that Sayyidna Musa (علیہ السلام) just did not have it. On the other side, Allah Ta` ala had blessed Sayyidna Musa (علیہ السلام) with an intense desire to seek knowledge. As soon as he smelt the scent of knowledge elsewhere too, knowledge that he did not have, he was all set to go for it traveling like an ardent student and it was Allah Ta` ala Himself that he turned to for the ad-dress of that servant of His (al-Khadir علیہ السلام). Now, there is something worth pondering at this point. Had it been the will of Allah Ta’ ala that Musa (علیہ السلام) should meet al-Khadir (علیہ السلام) at that spot, He could have arranged that easily. Or, if Sayyidna Musa (علیہ السلام) himself was Divinely destined to travel, he could have been given a clear address to help him reach it without any botheration. But, what happened here was different. The address given to him was far from being precise - 'when you reach the place where the dead fish stirs and disappears, that will be the place you will find Our servant.'
God continuously manages the affairs of this world through His angels. Since this system is not visible to man, he does not fully grasp its secrets. He harbours different types of doubts due to his deficient knowledge. To remedy this, God has given the opportunity for indirect observation to certain of His selected subjects. He has arranged for them to observe His invisible world, so that they should see with their own eyes its wisdom and make others aware of it. Here, the incident relating to Moses is a similar type of exceptional event, by means of which he was given a glimpse of God’s invisible system. Moses is said to have undertaken this journey between Egypt and Sudan along with one young disciple Yusha‘ bin Nun (Joshua). By way of a sign, God had asked him to proceed on his journey until he reached a place where two seas came together. He was told that there he would find somebody (perhaps an angel in the shape of a human being), whom he was asked to accompany.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.