Rashi on Numbers 21:24/רש"י על במדבר כ״א:כ״ד

1כי עז FOR [THE BOUNDARY OF THE CHILDREN OF AMMON] WAS STRONG — And what constituted its strength? The warning of the Holy One, blessed be He, who had said to them (the Israelites), (Deuteronomy 2:19), Harass them not, etc.”.