Numbers 21:13 in the World English Bible
From there they traveled, and encamped 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.
Context
12 From thence they removed, and pitched in the valley of Zared.
13 From thence they removed, and pitched on the other side of Arnon, which is in the wilderness that cometh out of the coasts of the Amorites: for Arnon is the border of Moab, between Moab and the Amorites.
14 Wherefore it is said in the book of the wars of the LORD, What he did in the Red sea, and in the brooks of Arnon,