Chafetz Chaim, Part One, The Prohibition Against Lashon Hara, Principle 5
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אִסוּר סִפּוּר לָשׁוֹן הָרָע, שֶׁאָסְרָה הַתּוֹרָה לְסַפֵּר בִּגְנוּת חֲבֵרוֹ, אֲפִלּוּ עַל אֱמֶת הוּא, אֲפִלּוּ בִּיחִידִי, (יא) וְכָל שֶׁכֵּן לְסַפֵּר בִּשְׁנַיִם דְּבַר גְּנוּת עַל חֲבֵרוֹ דְּאָסוּר, וְהֶעָוֹן הוּא יוֹתֵר גָּדוֹל מִן הַמְסַפֵּר בִּיחִידִי, כִּי יַאֲמִינוּ הָעָם יוֹתֵר, וְיִהְיֶה לְבוּז בְּעֵינִיהֶם בִּשְׁמִיעָתָם אֶת הַדְּבָרִים מִפִּי שְׁנַיִם. וּבְכָל מָקּוֹם שֶׁנִּכְתֹּב סְתַם אִסוּר לָשׁוֹן הָרָע, הַכַּוָּנָה הִיא בְּכָל גַּוְנִי {בכל האפנים}, לְבַד אִם נְפָרֵשׁ בְּהֶדְיָא {בּפֵרוּשׁ}.
The issur of speaking lashon hara, which the Torah forbade in speaking demeaningly of one's friend, applies even if it be true, and even in private; how much more so is it forbidden to speak demeaningly of one's friend before two. The sin is greater than doing so before one, for people will more readily believe it, and he will be more greatly scorned in their eyes if they hear it from two. And whenever the issur of lashon hara is mentioned in general, the intent is in all [eight] modes, unless we indicate otherwise.