Numbers 21:13-15

From there they went and camped across the Arnon, in the desert just inside the Amorite country. The Arnon is the border between the Moabites and the Amorites. That is why the Book of the Wars of the LORD says: “… and Waheb in Suphah, and the ravines, the Arnon, and the slopes of the ravines that lead to the settlement of Ar. These places are at the border of Moab.”