Numbers 32:28-30

28So Moses gave command concerning them to Eleazar the priest, to Joshua the son of Nun, and to the chief fathers of the tribes of the children of Israel.

29And Moses said to them: “If the children of Gad and the children of Reuben 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 as a possession.

30But if they do not cross over armed with you, they shall have possessions among you in the land of Canaan.”