Numbers 1:1-3

And the Lord said to Moses in the waste land of Sinai, in the Tent of meeting, on the first day of the second month, in the second year after they came out of the land of Egypt, Take the full number of the children of Israel, by their families, and by their fathers' houses, every male by name; All those of twenty years old and over, who are able to go to war in Israel, are to be numbered by you and Aaron.