Numbers 32:28-30

28So Moses gave orders about them to Eleazar the priest, to Joshua son of Nun, and to the family leaders of the tribes of Israel. 29And Moses said to them, “If the Gadites and Reubenites cross the Jordan with you, with every man armed for battle before the LORD, and the land is subdued before you, then you are to give them the land of Gilead as a possession. 30But if they do not arm themselves and go across with you, then they must accept their possession among you in the land of Canaan.”