Numbers 21:13-15

From there they journeyed on and camped on the other side of [the river] Arnon, which is in the wilderness that extends from the boundary of the Amorites; for [the river] Arnon is the boundary of Moab, between Moab and the Amorites. That is why it is said in the Book of the Wars of the LORD: “Waheb in Suphah, And the wadis of the Arnon [River], And the slope of the wadis That stretches toward the site of Ar And leans to the border of Moab.”