2 Chronicles 20:10, 23

And now behold, the men of Ammon and Moab and Mount Seir, whom you would not let Israel invade when they came from the land of Egypt, and whom they avoided and did not destroy

For the men of Ammon and Moab rose against the inhabitants of Mount Seir, devoting them to destruction, and when they had made an end of the inhabitants of Seir, they all helped to destroy one another.