Numbers 32:28-30

So Moses issued orders for them to Eleazar the priest, Joshua the son of Nun, and the heads of the ancestral tribes of the People of Israel. Moses said, “If the families of Gad and Reuben cross the Jordan River with you and before GOD, all armed and ready to fight, then after the land is secure, you may give them the land of Gilead as their inheritance. But if they don’t cross over with you, they’ll have to settle up with you in Canaan.”