Hilchos Lashon Hara chapter 7, section 3

Just as one may not believe lashon hara said by one person, so one may not believe it when said by two or more. First, their relating lashon hara gives them the status of wicked people who are suspected of lying and changing and adding facts. Even if they speak of something that does not make them wicked if it is true, one may still not believe them because two witnesses are only believed in a beis din. Outside a beis din they do not have the status of false witnesses if they lie, and are only considered as motzi shem ra. However, one may suspect the story is true even if it was said by one person