Numbers 21:11-15

Then they journeyed from Oboth and camped at Iye-abarim, in the wilderness which is opposite Moab, to the east.

From there they set out and camped in Wadi Zered.

From there they journeyed and camped on the other side of the Arnon, which is in the wilderness that comes out of the border of the Amorites; for the Arnon is the border of Moab, between Moab and the Amorites.

For that reason it is said in the Book of the Wars of the Lord, “Waheb in Suphah, And the wadis of the Arnon,

And the slope of the wadis That extends to the site of Ar, And leans to the border of Moab.”