Hilchos Lashon Hara chapter 8, section 7

Know also: if a person is known in the town as wicked because of sins that make it allowed to speak against him, the same rule applies. He must hear about the sins himself.

Who counts as known for doing sins? Someone whom the people of the town all agree is wicked, so that there is no doubt, because of the bad reports that are always coming out about him. But if there was only a rumor once, it is forbidden to rely on it to speak against him. It is even forbidden to decide it in one's heart.