Numbers 32:31 (KJV)

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, and the land shall be subdued before you; then ye shall give them the land of Gilead for a possession:

But if they will not pass over with you armed, they shall have possessions among you in the land of Canaan.

And the children of Gad and the children of Reuben answered, saying, As the Lord hath said unto thy servants, so will we do.

We will pass over armed before the Lord into the land of Canaan, that the possession of our inheritance on this side Jordan may be ours.

And Moses gave unto them, even to the children of Gad, and to the children of Reuben, and unto half the tribe of Manasseh the son of Joseph, the kingdom of Sihon king of the Amorites, and the kingdom of Og king of Bashan, the land, with the cities thereof in the coasts, even the cities of the country round about.