מדרש אויף במדבר 3:40

“The Lord said to Moses: Count every firstborn male of the children of Israel from one month old and above, and take the number of their names” (Numbers 3:40).
“The Lord said to Moses: Count every firstborn male of the children of Israel…” – that is what is written: “Since you were precious in My eyes, you were honored…” (Isaiah 43:4). The Holy One blessed be He said to Jacob: Jacob, you are exceedingly precious in My eyes, as, as it were, I have affixed your visage to my Throne, and the angels laud Me in your Name and say: “Blessed is the Lord, God of Israel, from eternity to eternity” (Psalms 41:14). That is, “since you were precious in My eyes, you were honored…”
Another matter: “Since you were precious in My eyes, you were honored” – the Holy One blessed be He said to Jacob: You are exceedingly precious in My eyes, as, as it were, I and My ministering angels emerged to greet you upon your departure to go to Padan Aram and upon your return. When you departed, what is written: “Jacob departed.… he encountered the place.… he dreamed, and behold, a ladder…[and behold, the angels of God].… And behold, the Lord stood over him… (Genesis 28:10–13). Rabbi Hoshaya said: Happy is one born of a woman who saw this, the King and His entourage standing over him and protecting him. From where is it derived upon his return? It is as it is stated: “Jacob went on his way, [and the angels of God encountered him]” (Genesis 32:2). Those are the angels; the Divine Presence, from where is it derived? “God appeared to Jacob again, upon his return [from Padan Aram]” (Genesis 35:9). That is, “since you were precious in My eyes, you were honored.”
Another matter: “Since you were precious in My eyes…” – the Holy One blessed be He said to Jacob: You are precious in My eyes, as for all the nations of the world, I did not provide a census, but for you, I provided a census. This is analogous to a king who had many granaries and they were all foul and filled with black kernels. He was not particular about their number. He had one granary that he saw that it was fine. He said to a member of his household: ‘Those granaries are foul and filled with black kernels; therefore, do not be particular about their number. But ascertain how many kor are in this one, how many sacks, how many measures are in it.’ So, the king, this is the King of kings, the Holy One blessed be He. The granary, this is Israel: “It is like My threshing floor, the product of my granary” (Isaiah 21:10). Likewise, it says: “Israel is sacred to the Lord, the first of His crop” (Jeremiah 2:3). The member of his household, this is Moses, as it is stated: “Not so My servant Moses; in all My house he is faithful” (Numbers 12:7). The Holy One blessed be He said to Moses: ‘The idol worshippers are foul,’ as it is stated: “Peoples will be as burnings of lime, cut thorns…” (Isaiah 33:12); ‘therefore, do not be particular about their number. But Israel, they are righteous; their groups are all wheat,’ as it is stated: “Your people, they are all righteous” (Isaiah 60:21). Likewise it says: “All of you is fair, my love, and there is no blemish in you” (Song of Songs 4:7). ‘That is why you shall be particular about their number.’ So, Moses did; he counted them, how many kor are in it, as it is stated: “Take a census of the entire congregation of the children of Israel” (Numbers 1:2); how many sacks: “its host and those counted” (Numbers 2:4); how many measures, as it is stated: “Count every…male.”

Another matter: “Count every firstborn male…” – that is what is written: “They are sixty queens…[and young women without number].… One is my perfect dove…” (Song of Songs (6:8–9). This is analogous to a person who had merchandise of glass jewels, and he would take them out to market, and he would not notice their number as he would not count when taking them out. He would enter to place them, and he would not count when placing them, as he would not scrutinize them because they were made of glass. But he had one batch of merchandise of fine gems, and he would take it and count it when taking it out and count it when placing. So, as it were, the Holy One blessed be He said: For the nations of the world, I did not provide a census. Why? It is because they are of no significance to Me, as it is stated: “All the nations are as nothing before Him” (Isaiah 40:17). But you are My children, as it is stated: “Who have been carried from birth…” (Isaiah 46:3). That is why He counts them every moment. That is why it is stated: “Count every firstborn male…” That is, “they are sixty queens…[and young women without number].… One is my perfect dove…” – these are Israel.

“From one month old and above” – we learned there: A baby who was born in the eighth month, one may not desecrate Shabbat for his sake, one may not cut his umbilical cord, one may not carry him from place to place, but rather, his mother leans over him and nurses him. One who carries him is like carrying a stone on Shabbat. If he was born in the seventh month and it is uncertain whether he was born in the eighth month, one may not desecrate Shabbat for his sake, one may not cut his umbilical cord, one does not bury his placenta, and one does not carry him from place to place. But if it is certain that he was born in the seventh month, he is alive; one may desecrate Shabbat for his sake. One cuts his umbilical cord and buries his placenta so that the infant will not be cold, and one carries him from place to place.¹From Tosefta, Shabbat 16:4. It contains only a short reference to the laws mentioned here. Why may one desecrate the Sabbath for one born in the seventh month? It is because he is viable. But one born in the eight month, his months of gestation were not completed and it is not viable.²There was a belief in the ancient world that a baby born after seven months was more viable than a baby born after eight months. Therefore, one may not desecrate Shabbat for its sake. They³The scholars of the nations of the world. asked Rabbi Abbahu: From where is it derived that one born in the seventh month is viable? I will bring an allusion for you from your own language: Live [zeta] – seven, go [eta] – eight.⁴The letter zeta represents seven in Greek, and it connotes life in several expressions. The letter eta represents eight and means go, which is interpreted as a reference to death.
Which is one born in the eighth? It is one whose nails and hair are not complete. Rabban Shimon ben Gamliel says: Anyone who does not live thirty days, his months of gestation were not completed, but it is stillborn. Why? The opinion of Rabban Shimon ben Gamliel was based on the words of the Torah, as the Omnipresent commanded to count the firstborn in order to redeem them, only after thirty days. That is what is written: “Count every [firstborn] male [of the children of Israel from one month old and above].”
“And take the number of their names” – He lifted up their heads, because the firstborn incurred liability to be executed due to the act of the calf.⁵The verse refers to the count of the firstborn as "lifting up their heads" to indicate that on the one hand they were being honored by being counted separately, but on the other hand they had been liable for having their heads removed, i.e., execution, for participating in the sin of the Golden Calf.