Numbers 21:21-25, 35

Israel sent messengers to say to Sihon king of the Amorites: “Let us pass through your country. We will not turn aside into any field or vineyard, or drink water from any well. We will travel along the King’s Highway until we have passed through your territory.” But Sihon would not let Israel pass through his territory. He mustered his entire army and marched out into the wilderness against Israel. When he reached Jahaz, he fought with Israel. Israel, however, put him to the sword and took over his land from the Arnon to the Jabbok, but only as far as the Ammonites, because their border was fortified. Israel captured all the cities of the Amorites and occupied them, including Heshbon and all its surrounding settlements.

So they struck him down, together with his sons and his whole army, leaving them no survivors. And they took possession of his land.