Numbers 1:18-20

18 and they assembled all the congregation together on the first day of the second new moon. And they declared their ancestry by clans, by their fathers’ houses, according to the number of names, from twenty years old and above, each one head by head.

19 As יהוה commanded Mosheh, so he registered them in the Wilderness of Sinai.

20 And the children of Re’uḇĕn, Yisra’ĕl’s first-born, their genealogies by their clans, by their fathers’ house, according to the number of names, every male head by head, from twenty years old and above, everyone going out to the army,