Chafetz Chaim, Part One, The Prohibition Against Lashon Hara, Principle 9
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הָאוֹמֵר לַחֲבֵרוֹ (טו) אֵיזֶה דָּבָר, אָסוּר לוֹ לְסַפֵּר אַחַר כָּךְ לַאֲחֵרִים, אֶלָּא אִם כֵּן נָתַן לוֹ רְשׁוּת עַל זֶה, וְדַוְקָא אִם הוּא דָּבָר דְּלֵית בֵּה לִישָׁנָא בִּישָׁא {שאין בו לשון הרע}.
Chofetz Chaim, Silverstein translation
If one tells his friend anything, he is forbidden to repeat it to others, unless he gave him permission to do so. And this, only if it contains no lashon hara.