Chafetz Chaim, Part One, The Prohibition Against Lashon Hara, Principle 2
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עוֹד נִרְאֶה לִי דְּדַוְקָא (יד) בְּאוֹתָה הָעִיר, שֶׁשָּׁמַע בְּאַפֵּי תְּלָתָא, מֻתָּר לְגַלּוֹת מִשּׁוּם דְּחַבְרָךְ חַבְרָא וְכוּ', אֲבָל לֹא בְּעִיר אַחֶרֶת, אַף דְּשַׁיָּרוֹת מְצוּיוֹת מִזּוֹ לְזוֹ, וְעַיֵּן בִּבְאֵר מַיִם חַיִּים.
Chofetz Chaim, Silverstein translation
It also seems to me that only in that city in which the report was heard in the presence of three is it permitted to reveal it on the basis of "Your friend has a friend, etc.," but not in a different city, even if there were communication [lit., "caravans"] between the two. [See Be'er Mayim Chayim.]