Sforno on Genesis 18:31/ספורנו על בראשית י״ח:ל״א

1לא אשחית בעבור העשרים...בעבור העשרה. By destroying 3 out of the five cities the other two will not escape the retribution completely, seeing that the satellite towns are by definition largely dependent on the major urban center. Its destruction will cast its shadow on the satellite towns, as we know from Berachot 58, i.e. נתקללה בבל נתקללה בנותיה, when Babylon is cursed so are its satellite towns. (according to Rashi there: אוי לרשע אוי לשכניו, when the wicked experiences woes, so do his neighbours.”)