Numbers 21:13, 24

Then they moved out and camped on the far side of the Arnon River, in the wilderness adjacent to the territory of the Amorites. The Arnon is the boundary line between the Moabites and the Amorites.

But the Israelites slaughtered them with their swords and occupied their land from the Arnon River to the Jabbok River. They went only as far as the Ammonite border because the boundary of the Ammonites was fortified.