Targum for Numbers 21:28

for mighty words like fire go forth from the lips of the righteous, the masters of such thought, (calculation, heshbona,) and powerful merit like flames from those who are read and devoted in the law: their fire devoureth the foe and the adversary, who are reckoned before them as the worshippers of the idol altars in the valley of Arnona.

Because the men of their people like fire come out of Heshbon, making war as flames of fire from the city of Sihon: the kings of the Amoraee are slain, the villages of the Moabite cities are destroyed, and the priests are slaughtered who sacrificed before the idols of Arnona.