Numbers 32:29-32

And 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. But if they do not cross over armed with you, they shall have possessions among you in the land of Canaan.” Then the children of Gad and the children of Reuben answered, saying: “As the LORD has said to your servants, so we will do. We will cross over armed before the LORD into the land of Canaan, but the possession of our inheritance shall remain with us on this side of the Jordan.”