Numbers 1:18-20

New Revised Standard Version Catholic Edition

¹⁸ and on the first day of the second month they assembled the whole congregation together. They registered themselves in their clans, by their ancestral houses, according to the number of names from twenty years old and upward, individually, ¹⁹ as the Lord commanded Moses. So he enrolled them in the wilderness of Sinai.

²⁰ The descendants of Reuben, Israel’s firstborn, their lineage, in their clans, by their ancestral houses, according to the number of names, individually, every male from twenty years old and upward, everyone able to go to war: