Rashi on Genesis 18:24/רש"י על בראשית י״ח:כ״ד

1אולי יש חמשים צדיקים PERADVENTURE THERE BE FIFTY RIGHTEOUS — ten righteous men for each city for there were five localities concerned. Should You, however, say that the righteous cannot save the wicked—but why should You kill the righteous at all?