Numbers 21:13

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

Numbers 22:36

36When Balak heard that Balaam had come, he went out to meet him at the city of Moab, bon the border formed by the Arnon, at the extremity of the border.
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