Numbers 32:28-29

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

29Moses said to them, “If the sons of Gad and the sons of Reuben, everyone who is armed for battle, will cross with you over the Jordan in the presence of the Lord, and the land is subdued before you, then you shall give them the land of Gilead for a possession;