כִּי־אֵשׁ֙ יָֽצְאָ֣ה מֵֽחֶשְׁבּ֔וֹן לֶהָבָ֖ה מִקִּרְיַ֣ת סִיחֹ֑ן אָֽכְלָה֙ עָ֣ר מוֹאָ֔ב בַּעֲלֵ֖י בָּמ֥וֹת אַרְנֹֽן׃
For a fire is gone out of Heshbon, A flame from the city of Sihon; It hath devoured Ar of Moab, The lords of the high places of Arnon.
Rashi on Numbers
כי אש יצאה מחשבון FOR A FIRE WILL GO FORTH FROM HESHBON after Sihon captures it,
Rashbam on Numbers
ער מואב, the capital of the land of Moav was called Or.
Rabbeinu Bahya
כי אש יצאה מחשבון, ”for a fire has come forth from Cheshbon.” This is an allusion to Moabites who had dwelled in Chesbon and commenced a revolution against the local king.