Rashi on Numbers 21:27/רש"י על במדבר כ״א:כ״ז

1על כן (more lit., about this) — about this war which Sihon waged against Moab,

2יאמרו המשלים THEY THAT SPEAK IN PARABLES SAY — One of these was Balaam of whom it is said, (Numbers 23:7), "And he took up his parable”.

3המשלים THEY THAT SPEAK IN PARABLES — The plural המשלים refers to Balaam and his father Beor (see Rashi on Numbers 24:3). They said —

4באו חשבון “COME TO HESHBON”, — For Sihon had been unable to capture it, and he went and hired Balaam to curse it, and this is the meaning of what Balak said, (Numbers 22:6) “For I know (by what has already happened) that whomsoever thou blessest is blessed, [and whomsoever thou cursest is cursed]” (Midrash Tanchuma, Chukat 24).

5תבנה ותכונן IT WILL BE BUILT UP AND ESTABLISHED, [SIHON’S CITY] — Heshbon will be rebuilt under Sihon’s name to be his city.