Chafetz Chaim, Part One, The Prohibition Against Lashon Hara, Principle 2
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(א) אָסוּר לְסַפֵּר לָשׁוֹן הָרָע עַל חֲבֵרוֹ, אַף שֶׁהוּא אֱמֶת, אֲפִלּוּ בִּפְנֵי יָחִיד. וְכָל שֶׁכֵּן בִּפְנֵי רַבִּים. וְכָל שֶׁיִּתְרבּוּ הַשּׁוֹמְעִים, יִתְרבֶּה עֲוֹן הַמְסַפֵּר, מִפְּנֵי שֶׁחֲבֵרוֹ מִתְגַּנְּה יוֹתֵר עַל יְדֵי זֶה, שֶׁנִּתְפַּרְסֵם גְּנוּתוֹ בִּפְנֵי כַּמָּה אֲנָשִׁים, גַּם שֶׁעַל יְדֵי זֶה מַכְשִׁיל כַּמָּה אֲנָשִׁים בְּאִסוּר שְׁמִיעַת לָשׁוֹן הָרָע {הסכמת כל הפוסקים}.
It is forbidden to speak lashon hara against one's friend, even if it is true, even before one, and, more so, before many. And the more listeners, the greater the sin of the speaker; for his friend is more greatly demeaned thereby, his taint being publicized before several people. Also, in doing so, he makes several people go astray in the issur of listening to lashon hara.