Numbers 1:18-20

Young's Literal Translation

¹⁸ and all the company they assembled on the first of the second month, and they declare their births, by their families, by the house of their fathers, in the number of names from a son of twenty years and upward, by their polls,

¹⁹ as Jehovah hath commanded Moses; and he numbereth them in the wilderness of Sinai.

²⁰ And the sons of Reuben, Israel's first-born -- their births, by their families, by the house of their fathers, in the number of names, by their polls, every male from a son of twenty years and upward, every one going out to the host --