Chafetz Chaim, Part Two, The Prohibition Against Rechilut, Principle 8
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יֵשׁ הַרְבֵּה דְּבָרִים, שֶׁאֲסוּרִין מִטַּעַם אֲבַק רְכִילוּת. וַאֲבָאֵר בְּקִצּוּר אֵיזֶה עִנְיָנִים, וּמֵהֶם יָקִישׁ הַמַּשְׂכִּיל לְכָל כַּיּוֹצֵא בָּזֶה, כְּגוֹן, (א) הַמְסַפֵּר לַחֲבֵרוֹ, אֵיךְ שֶׁשָּׁאֲלוּ מֵאֶחָד אוֹדוֹתָיו, וְהֵשִׁיב עַל זֶה: שִׁתְקוּ, אֵינִי רוֹצֶּה לְהוֹדִיעַ מָה אֵרַע, וּמַה יִּהְיֶה, וְכָל כַּיּוֹצֵּא בָּזֶה, שֶׁמְסַפֵּר לוֹ, אֵיךְ שֶׁמִּדִּבְרֵי פְּלוֹנִי נִרְמַז עָלָיו שֶׁלֹּא כַּהֹגֶן, בִּכְלַל אֲבַק רְכִילוּת הוּא.
There are many things which are forbidden because of the "dust" of rechiluth. I shall explain some of them in short, and the intelligent reader will infer their parallels: one's telling his friend that they asked Ploni about him and he answered: "Hush! I don't want to mention what happened and what is going to happen!" All such things, where one tells him how Ploni's words intimated something ill of him are in the category of the "dust" of slander.