Numbers 32:29

And Moses said unto them,.... To Eleazar, Joshua, and the princes of the congregation:

if the children of Gad and the children of Reuben will pass with you over Jordan, every man armed to battle before the Lord; as they have promised they will:

and the land shall be subdued before you; which must be done before their return:

then ye shall give them the land of Gilead for a possession; which, no doubt, included that of Jazer too, since Jaazer, which is the same, is after mentioned as one of the cities built by the children of Gad,

Numbers 32:35.