Numbers 32:28-30

28So Moses gave the command concerning them to Eleazar the priest, and to Joshua the son of Nun, and to the leaders of the fathers’ households of the tribes of the Israelites.

29Moses said to them, “If the sons of Gad and the sons of Reuben will cross over the Jordan with you, every man armed for battle before the Lord, and the land is subdued before you, then you shall give them the land of Gilead [east of the Jordan River] as a possession;

30but if they will not cross over with you armed, they shall have possessions among you in the land of Canaan [and surrender their right to their land east of the Jordan River].”