Numbers 21:24, NLT

24But the Israelites slaughtered them with their swords and occupied their land from the Arnon River to the Jabbok River. They went only as far as the Ammonite border because the boundary of the Ammonites was fortified.*21:24 Or because the terrain of the Ammonite frontier was rugged; Hebrew reads because the boundary of the Ammonites was strong.