Midrasch zu Jechezkiel 14:19

Hence they said: If there be fifty righteous in the world, the world exists through their righteousness. (Abraham) arose and began to beseech (God), and made supplication before Him until he brought (the number down to) ten. Hence (the sages said): (When there are) ten people in a place, the place is delivered by their righteousness, as it is said, "And he said, I will not destroy it for the sake of the ten" (Gen. 18:32).