Numbers 1:18-20

Darby Translation

¹⁸ and gathered the whole assembly together on the first of the second month. And they declared their pedigrees after their families, according to their fathers' houses, by the number of the names, from twenty years old and upward, according to their polls.

¹⁹ As Jehovah had commanded Moses, so he numbered them in the wilderness of Sinai.

²⁰ And the sons of Reuben, Israel's eldest son, their generations, after their families, according to their fathers' houses, by the number of the names, according to their polls, every male from twenty years old and upward, all that went forth to military service: