Numbers 32:16-19

16But the people of Reuben and Gad approached Moses and said, “We will build sheepfolds here for our flocks and [walled] cities for our children,

17but we will be armed and ready to go to war before the [other tribes of the] Israelites, until we have brought them to their place, while our children live in the fortified cities because of the inhabitants of the land.

18We will not return to our homes until every one of the [other] sons of Israel has taken possession of his inheritance.

19For we will not inherit with them on the west side of the Jordan and beyond, because our inheritance has come to us on this side of the Jordan [River] toward the east.”