Chafetz Chaim, Part One, The Prohibition Against Lashon Hara, Principle 7
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אִסוּר קַבָּלַת לָשׁוֹן הָרָע הוּא אֲפִלּוּ אִם הַמְסַפֵּר מְסַפֵּר אֶת הַדְּבָרִים בְּפִרְסוּם (א) לִפְנִי כַּמָּה אֲנָשִׁים, אַף עַל פִּי כֵן אֵין לְהַחְלִיט מֵחֲמַת זֶה, שֶׁהַדָּבָר אֱמֶת, רק יֵשׁ לְהַשּׁוֹמְעִים לָחוּשׁ לָזֶה וְלַחֲקֹר אֶת הַדָּבָר, וְאִם יִתְבָּרֵר לָהֶם, שֶׁהַדָּבָר אֱמֶת, יוֹכִיחוּהוּ עַל זֶה.
The issur of accepting lashon hara obtains even if the speaker speaks it in public, before many people. Still, it is not to be concluded because of this that it is true, but the bearers must suspect [that it might be true] and probe the matter. And if it becomes clear to them that it is true, they should reprove him [the object of the lashon hara] for it (i.e., for what he has done or said).