Chafetz Chaim, Part One, The Prohibition Against Lashon Hara, Principle 7
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וְאִם אֶחָד בָּא אֶצְּלוֹ וּמְסַפֵּר לוֹ אֶת עִנְיָנָיו וּבְתוֹךְ סִפּוּרוֹ סִפֵּר לוֹ אֵיזֶה דָּבָר, שֶׁהוּא גְּנוּת לְעַצְמוֹ וְלַחֲבֵרוֹ, אֵינוֹ מֻתָּר לְהַאֲמִינוֹ, רַק עַל עַצְּמּוֹ (יא) וְלֹא עַל חֲבֵרוֹ.
Chofetz Chaim, Silverstein translation
And if one comes to him and tells him of his affairs, and includes something demeaning both to himself and to his friend, he is permitted to believe only what applies to himself and not what applies to his friend.