Midrash sa 1 Samuel 17:28: Shemot Rabbah, Ein Yaakov (Glick Edition) & Midrash Tanchuma

Another matter, “Moses was herding” – that is what is written: “Every word of God is pure” (Proverbs 30:5); the Holy One blessed be He does not bestow greatness upon a person until He tests them with a minor matter, and only then does He elevate him to prominence. There were two preeminently great ones whom the Holy One blessed be He tested with a minor matter, and they were found trustworthy and He elevated them to prominence.
He tested David with the flocks, and he led them only to the wilderness to distance them from robbery, as Eliav⁴David’s older brother. said to David: “With whom did you leave those few sheep in the wilderness?” (I Samuel 17:28). This teaches that David observed [the halakha cited in] the mishna (Bava Kamma 79a–79b): One may not raise a small domesticated animal in the Land of Israel.⁵For that reason David shepherded his flock in the wilderness. The Holy One blessed be He said to him: ‘You have been found trustworthy with the flocks, come and herd My flock,’ as it is stated: “From the suckling ewes He brought him” (Psalms 78:71).
Likewise, regarding Moses it says: “He led the flock deep into the wilderness,” to remove them from theft [by grazing in fields belonging to others], and the Holy One blessed be He took him to herd Israel, as it is stated: “You led Your people like a flock in the hand of Moses and Aaron” (Psalms 77:21).

(Ib. b) R. Juda said, in the name of Rab: "Whoever is arrogant, if he be a sage, his wisdom will depart from him: and if he be a prophet, his prophecy will depart from him. That his wisdom will depart from him, if he be a sage, can be inferred from the case of Hillel, for the master said [above] , 'he began to reproach them,' and immediately after, when they asked him a law, he said, 'I heard, but I have forgotten it.' If he be a prophet, his prophecy will depart from him; this may be gathered from the case of Deborah [the prophetess]; for it is written (Jud. 5, 7) Desolate were the open towns in Israel; they were desolate, until that I arose, Deborah, thus I arose a mother in Israel; and it is written (Ib. ib. 12) Awake, awake, Deborah, awake, awake, utter a song, [which signifies that the prophecy left her and she had to awaken herself]." R. Simon b. Lakish said: "A man who becomes excited, if he be a sage, will have his wisdom depart from him; and if he be a prophet, will have his prophecy depart from him. That his wisdom will depart from him, if he be a sage, we infer from Moses, concerning whom it is written (Num. 31, 41) And Moses was excited against the officers of the host; and [after this] it is written, And Elazar the Priest said, etc. From this it may be inferred that the law escaped Moses' memory and therefore Elazar had to say it. That his prophecy will depart from him, if he be a prophet, we infer from Elisha, concerning whom it is written (II Kings 3, 14) Were it not that I regard the presence of Jehoshaphat, the King of Judah, I would not look toward thee, nor see thee; and [after this] it is written. But now bring me a musician; and it came to pass, when the musician played, and the inspiration of the Lord came upon him, [which shows that he had to seek the inspiration]." R. Mani b. Patish said: "A man who becomes excited will be removed from greatness even if it had been predestined for him by Heaven. Whence do we deduce this? From Eli'ab concerning whom it is written (I Sam. 17, 28) And Eli'ab's anger was incited against David, and he said, etc.; and when Samuel went to anoint a king, from among the sons of Jesse, concerning all others [who were rejected] it is written (Ib. 17, 8-10) This one hath the Lord not chosen [in the past tense]. But concerning Eli'ab, it is written (Ib. ib., 7) But the Lord said unto Samuel, 'Regard not his appearance, nor the height of his stature, because I have rejected him,' [in the present tense]. From which it may be inferred that I rejected him now although I had intended to choose him."

Now Moses was keeping the flock (Exod. 3:1). Scripture states elsewhere: Every word of God is tried; He is a shield unto them that take refuge in Him (Prov. 30:5). The Holy One, blessed be He, does not confer greatness upon a man until He tests him in lesser things. Only then does He elevate him to greatness. Two great men in the world were tested and found to be faithful before He raised them to positions of greatness. He tested David by means of a sheep. What did David do? He led the sheep into the desert to keep them from feeding from the field of others, and that is why his brother Eliab said to him: With whom hast thou left those few sheep in the wilderness? (I Sam. 17:28). The Holy One, blessed be He, said to him: Thou hast been found trustworthy with regard to sheep, and so now I shall entrust My flock to you that you may shepherd them, as it is written: Thou shalt shepherd My people Israel (I Chron. 11:2). And Scripture likewise says: From following the ewes that give suck He brought him to be shepherd over Jacob, His people, and Israel, His inheritance (Ps. 78:71).