מדרש אויף במדבר 4:37

“And the princes of the congregation [counted] the sons of the Kehatites.” Why are the princes necessary here? Is it not so, that the Holy One blessed be He did not say to Moses that the princes of the congregation should be with him in the census of the Levites? Why did he do so? Rather, this is what Moses said: ‘Since the princes were with me in the census of all Israel, just as it says: “And with you shall be [a man for each tribe, each is the head of his patrilineal house]” (Numbers 1:4), I will not cause them anguish and I will take them with me for the census of the Levites.’
From where is it derived that this is so? You find that after the census of Israel it is written: “These are the counted, that [Moses and Aaron, and the princes of Israel] counted” (Numbers 1:44), but after the census of the Levites, you do not find that the princes are mentioned there, but rather, “whom Moses and Aaron counted” (Numbers 4:37). It is to teach you that the princes of Israel did not count the Levites at the directive of divine speech. But rather Moses took them with him in order to accord them honor.

“From thirty [years old and above]” (Numbers 4:35); from here they said: Thirty years old for strength (Mishna Avot 5:21).
“Their counted, by their families, was two thousand seven hundred and fifty” (Numbers 4:36).
“Their counted, by their families,” this is what they said, that the Ark would kill of the sons of Kehat, as when he counted them from one month old, they were eight thousand six hundred, but when he counted them from thirty years old, not even one third of them were found.
“These are the counted of the Kehatite families, all who served in the Tent of Meeting, whom Moses and Aaron counted according to the directive of the Lord at the hand of Moses” (Numbers 4:37).
“These are the counted of the Kehatite families,” this is what they said, that the princes did not count with them at the directive of the divine speech, as the princes are not mentioned here.
“According to the directive of the Lord at the hand of Moses,” they acted at the directive of the Lord, just as the Holy One blessed be He said to Moses that Aaron should count the sons of Kehat with him, as it is stated: “The Lord spoke to Moses and Aaron saying: Take a census of [the sons of Kehat]” (Numbers 4:1–2). That is why it is stated: “At the hand of Moses,” as Moses said to Aaron, because Aaron was excluded from all the statements in the Torah that were stated to Moses.

“And the counted of the sons of Gershon, by their families, and by their patrilineal house” (Numbers 4:38).
“And the counted of the sons of Gershon,” just as he counted the sons of Kehat at the directive of the Lord, so he counted the sons of Gershon.
“From thirty years old and above until fifty years old, everyone who enlisted for duty, for work in the Tent of Meeting” (Numbers 4:39).
“From thirty years old,” in the Tent of Meeting they were disqualified from fifty years and onward, but when they entered the Land of Israel, they were disqualified only on the basis of voice.¹⁷Only if their voices were not fit to join the Levites in their song in the Temple service.
“Their counted by their families, by their patrilineal house, were two thousand and six hundred and thirty” (Numbers 4:40).
“Their counted,” this is what they said, that the sons of Gershon, because they were not among the bearers of the Ark, that is why they were not lacking two-thirds from the first census, but rather, they were one hundred and thirty more than one-third.
“These are the counted of the families of the sons of Gershon, all who worked in the Tent of Meeting, whom Moses and Aaron counted according to the directive of the Lord” (Numbers 4:41).
“These are the counted of the families….” Why, in the census of the Kehatites, are “the sons of Kehat” not mentioned, just as it is stated regarding the sons of Gershon and the sons of Merari, but “the Kehatite [haKehati]” is written? Because their census was for the purpose of bearing the Ark, the Holy One blessed be He appended His name to theirs, so they would not be eradicated; a heh at the beginning and a yod at the end. That is ya; so as to say that ya will deliver them from death, to realize what is stated: “To deliver them from death” (Psalms 33:19).
“Whom Moses and Aaron counted according to the directive of the Lord.” Why is it not stated in the census of the sons of Gershon “at the hand of Moses,” as it is stated in the census of the sons of Kehat? It is because regarding the sons of Kehat, the Holy One blessed be He said to Moses that Aaron should count with Moses. The Holy One blessed be He mentioned Aaron at the beginning of the census with the intent that he should count all the Levites with Moses. However, when Moses heard that the Holy One blessed be He mentioned Aaron in the census of the sons of Kehat but did not mention him subsequently, neither in the census of the sons of Gershon nor in the census of the sons of Merari, Moses thought to himself and said: ‘This is why the Holy One blessed be He said to me, that Aaron should count the sons of Kehat with me; it is because all their burden and all their service is performed under his direction. But the service of the sons of Gershon and the sons of Merari, that are not performed under his direction, He does not want him to count with me.’ That is why he said to Aaron that he should count the sons of Kehat with him, in the name of the Holy One blessed be He, and that is why it is written in the census of the sons of Kehat: “According to the directive of the Lord at the hand of Moses” (Numbers 4:37). But when he came to count the sons of Gershon, Moses did not say to Aaron in the name of the Holy One blessed be He that he should count the sons of Gershon with him, but rather, he accorded him honor because he was his older brother, and took him with him and included him with him for the census. That is why “at the hand of Moses” is not stated. Even though it was the intent of the Holy One blessed be He that Aaron should count all the Levites with him, and it was with that intent that He mentioned Aaron at the beginning of the census, because Moses did not say to Aaron that he should count the sons of Gershon with him in the name of the Holy One blessed be He, he omitted here “at the hand of Moses.”