Numbers 32:29

and Moses said unto them, If the children of Gad and the children of Reuben will pass with you over Jordan, every man armed to battle, before the Lord, who still accompanied the host, and the land shall be subdued before you, then ye shall give them the land of Gilead for a possession, which they were now holding only in a temporary and tentative manner, subject to the fulfillment of their promise;