Numbers 32:28-30

28Then Moses gave orders about them to Eleazar the priest and Joshua son of Nun and to the family heads of the Israelite tribes.

29He said to them, “If the Gadites and Reubenites, every man armed for battle, cross over the Jordan with you before the Lord, then when the land is subdued before you, you must give them the land of Gilead as their possession.

30But if they do not cross over with you armed, they must accept their possession with you in Canaan.”