Hilchos Lashon Hara chapter 6, section 7

. Just as one may not believe lashon hara, so too, even in a case where one knows the story is true but the speaker does not take possibly alleviating factors into account and we know that it is a mitzvah to judge a person charitably (as stated in Shavuos 30a and this is a positive mitzvah according to many authorities) and the listener agrees to the speaker’s version of what happened, he not only transgresses, You shall judge your fellow charitably, but is also considered a believer of lashon hara for accepting the worse version of the story.