Numbers 1:16-20

These are the men who were called from 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 on the first day 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 counted them in the wilderness of Sinai.

Now the sons of Reuben, Israel’s firstborn, their descendants by their families, by their fathers’ households, according to the number of names, head by head, every male from twenty years old and upward, whoever was able to go to war,