Numbers 32:28-30

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

29Moses told them, “If the Gadites and Reubenites cross the Jordan with you, every man in battle formation before the Lord, and the land is subdued before you, you are to give them the land of Gilead as a possession.

30But if they don’t go across with you in battle formation, they must accept land in Canaan with you.”