Rashi on Numbers 21:28/רש"י על במדבר כ״א:כ״ח

1כי אש יצאה מחשבון FOR A FIRE WILL GO FORTH FROM HESHBON after Sihon captures it,

2אכלה ער מואב IT WILL CONSUME AR OF MOAB — The name of that province was called Ar in the Hebrew language and Lechayath (see the Targum Onkelos) in the Aramaic language.

3ער מואב means AR OF MOAB.