Numbers 1:1-3

And the LORD spake unto Moses in the wilderness of Sinai, in the tabernacle of the congregation, on the first {day} of the second month, in the second year after they were come out of the land of Egypt, saying, Take ye the sum of all the congregation of the children of Israel, after their families, by the house of their fathers, with the number of {their} names, every male by their polls; From twenty years old and upward, all that are able to go forth to war in Israel: thou and Aaron shall number them by their armies.