Numbers 21:13-15

From there they moved 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, The brooks of the Arnon, And the slope of the brooks That reaches to the dwelling of Ar, And lies on the border of Moab.”