Numbers 3:40
And the LORD said unto Moses, Number all the firstborn of the males of the children of Israel from a month old and upward, and take the number of their names. And the LORD said unto Moses, Number all the firstborn of the males of the children of Israel from a month old and upward, and take the number of their names.
The command stands within the detailed census of Israel, which displays divine care in counting the people tribe by tribe under appointed inspectors. It gives the definite number by name of the firstborn males of Israel from a month old and upward. That counting belongs to the description of the divisions of the sons of Levi.
It is part of God's assignment of the tabernacle and its service to the Levites, with directions for arranging the camp of Israel and dividing the duties of the Levites around the tabernacle. It belongs to the legal establishment of the Aaronic priesthood, which was assigned to Levi to serve until the type met its antitype in Christ. The numbering of the firstborn males is recorded in connection with the transfer of the priesthood to the Levites and defines the relation of the firstborn to the Levites.
Even after that transfer the Israelites were still required to redeem their firstborn, and the redemption money went to the Levites. The verse is one of the definite numberings of men of a certain age in Numbers 1-3, to be referred back to on reaching Numbers 26 at the close of the forty years in order to note how the tribes have gained and lost. It belongs to the levitical law given to the children of Israel, distinct from the absolute and perpetual law of God.