Numbers 21:13-15

From there they set out and camped on the other side of the Arnon, which is in the wilderness that extends from the border of the Amorites, for the Arnon is the border of Moab, between Moab and the Amorites. Therefore it is said in the Book of the Wars of the LORD, “Waheb in Suphah, and the valleys of the Arnon, and the slope of the valleys that extends to the seat of Ar, and leans to the border of Moab.”