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 ) .