Numbers 21:13, 24

They set out from there and camped alongside the Arnon, which is in the wilderness extending into Amorite territory. The Arnon is the border of Moab, between Moab and the Amorites.

Israel, however, put him to the sword and took over his land from the Arnon to the Jabbok, but only as far as the Ammonites, because their border was fortified.