Numbers 21:13-15

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. That is why the Book of the Wars of the LORD says: “…Zahab in Suphah and the ravines, the Arnon and the slopes of the ravines that lead to the settlement of Ar and lie along the border of Moab.”