Numbers 21:13-15

13From there they moved on and camped on the other side of the Arnon, in the wilderness that extends from the regions of the Amorites, for Arnon is the border of Moab, between Moab and the Amorites.

14This is why it is said in the Book of the Wars of the Lord, “Waheb in Suphah and the wadis, the Arnon

15and the slope of the valleys that extends to the dwelling of Ar, and falls off at the border of Moab.”