Numbers 1:3, 18, 20

You and Aaron are to count according to their divisions all the men in Israel who are twenty years old or more and able to serve in the army.

and they called the whole community together on the first day of the second month. The people registered their ancestry by their clans and families, and the men twenty years old or more were listed by name, one by one

From the descendants of Reuben the firstborn son of Israel: All the men twenty years old or more who were able to serve in the army were listed by name, one by one, according to the records of their clans and families.