Rashi on Genesis 18:26/רש"י על בראשית י״ח:כ״ו

1אם אמצא בסדום וגו' לכל המקום IF I FIND IN SODOM … FORGIVE ALL THE PLACE — all the cities, but because Sodom was the capital city of the district and the most important of all, Scripture subordinates the order cities to it.