Numbers 1:3-5

3 from twenty years old and upward, all who are able to go forth to war in Israel. Thou and Aaron shall number them by their armies.

4 And with you there shall be a man of every tribe, each one head of his fathers' house.

5 And these are the names of the men who shall stand with you. Of Reuben: Elizur the son of Shedeur.