Numbers 1:1-3

And Jehovah spake unto Moses in the wilderness of Sinai, in the tent of meeting, 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, by their families, by their fathers’ houses, according to the number of the 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 hosts.