Numbers 21:13-15
From there they removed, and pitched on the other side of Arnon, which is in the wilderness that comes out of the coasts of the Amorites: for Arnon is the border of Moab, between Moab and the Amorites.
Torrey's Topical Textbook—AKJV

Deuteronomy 2:24
Rise you up, take your journey, and pass over the river Arnon: behold, I have given into your hand Sihon the Amorite, king of Heshbon, and his land: begin to possess it, and contend with him in battle.
Torrey's Topical Textbook—AKJV