Numbers 1:18-20

Revised Standard Version

¹⁸ and on the first day of the second month, they assembled the whole congregation together, who registered themselves by families, by fathers’ houses, according to the number of names from twenty years old and upward, head by head, ¹⁹ as the Lord commanded Moses. So he numbered them in the wilderness of Sinai.

²⁰ The people of Reuben, Israel’s first-born, their generations, by their families, by their fathers’ houses, according to the number of names, head by head, every male from twenty years old and upward, all who were able to go forth to war: