Numbers 1:18-20

18 called together the whole community on the first day of the second month and registered all the people by clans and families. The names of all the men twenty years old or older were recorded and counted,

19 as the LORD had commanded. In the Sinai Desert, Moses registered the people.

20 The men twenty years old or older who were fit for military service were registered by name according to clan and family, beginning with the tribe of Reuben, Jacob's oldest son. The totals were as follows: