Numbers 1:1-3

1And Jehovah spoke to Moses in the wilderness of Sinai in the tent of meeting, on the first of the second month, in the second year after their departure from the land of Egypt, saying,

2Take the sum of the whole assembly of the children of Israel, after their families, according to their fathers' houses, by the number of the names, every male, according to their polls;

3from twenty years and upward, all that go forth to military service in Israel: ye shall number them according to their hosts, thou and Aaron.