Numbers 1:16-19

These men were the ones called from the congregation, the leaders of their fathers’ (ancestors’) tribes; they were the heads of thousands [the highest ranking officers] in Israel.” So Moses and Aaron took these men who were designated by name, and assembled all the congregation on the first day of the second month, and they registered by ancestry in their families (clans), 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.