Numbers 32:28-29

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

29And Moses said unto them, If the children of Gad and the children of Reuben pass with you over the Jordan, every one armed for battle, before Jehovah, and the land be subdued before you, then ye shall give them the land of Gilead for a possession;