Numbers 1:16-19

These are they who were called of the congregation, the leaders of their fathers’ tribes; they were the heads of divisions of Israel.” So Moses and Aaron took these men who had been designated by name, and they assembled all the congregation together on the first of the second month. Then they registered by ancestry in their families, by their fathers’ households, according to the number of names, from twenty years old and upward, head by head, just as the LORD had commanded Moses. So he numbered them in the wilderness of Sinai.