Allah tells us what Musa said to that learned man, who was Al-Khidr. He was one to whom Allah had given knowledge that He had not given to Musa, just as He had given Musa knowledge that He had not given to Al-Khidr.
قَالَ لَهُ مُوسَى هَلْ أَتَّبِعُكَ
(Musa said to him: "May I follow you...") This is a question phrased in gentle terms, with no sense of force or coercion. This is the manner in which the seeker of knowledge should address the scholar.
اتَّبَعَكَ
(I follow you) means, I accompany you and spend time with you.
عَلَى أَن تُعَلِّمَنِ مِمَّا عُلِّمْتَ رُشْداً
(so that you teach me something of that knowledge which you have been taught) meaning, teach me something from that which Allah has taught you so that I may be guided by it and learn something beneficial and do righteous deeds. At this point,
It is not permissible for an 'Alim’ of the 'Shari'ah' to patiently bear what is contrary to the Shari'ah
By saying: إِنَّكَ لَن تَسْتَطِيعَ مَعِيَ صَبْرًا وَكَيْفَ تَصْبِرُ عَلَىٰ مَا لَمْ تُحِطْ بِهِ خُبْرًا (You can never be able to keep patient while with me. And how would you keep patient over something your comprehension cannot grasp? - 67, 68), al-Khadir (علیہ السلام) was telling Sayyidna Musa (علیہ السلام) about the fact and the reason why he would be unable to keep patient with him. He knew nothing about the reality of the thing. What he meant was that the nature of knowledge given to him was different from the knowledge of Sayyidna Musa (علیہ السلام) ، therefore, things he did would appear to be objectionable in his sight, until he himself was to apprise him of their reality. Thus, the objections he would raise against such actions would be triggered by the dictates of his mission as a prophet.
Moses was told about a further sign, i.e. that when he reached the required destination, the fish kept for eating would go down into the water in a peculiar manner. And at a certain place this was exactly what happened. But the fish was with the disciple and he could not for some reason inform Moses about this incident. After proceeding a little further, Moses came to know of this incident. He immediately returned to that place and found Khidr there, the person for whom he had undertaken such a long journey. God had bestowed on this servant, Khidr, special knowledge and powers, so that he could bring certain extraordinary influence to bear upon the affairs of this world. In view of this knowledge, he used to act against the customary norms. That is why, on Moses request to be allowed to accompany him, he said that he Moses would not be able to bear it.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.