Chafetz Chaim, Part One, The Prohibition Against Lashon Hara, Principle 1
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וְדַע עוֹד, דַּאֲפִלּוּ אִם בְּתוֹךְ הַגְּנוּת, שֶׁגִּנָּה אֶת חֲבֵרוֹ, גִּנָּה אֶת עַצְּמוֹ (טו) גַּם כֵּן בָּזֶה הַגְּנוּת גּוּפָא, וַאֲפִלּוּ הִקְדִּים לְהִתְרַעֵם עַל עַצְּמוֹ בָּזֶה, אֲפִלּוּ הָכֵי {אף על פי כן} מִכְּלל דֵּילָטוֹרְיָא לָא נָפְקֵי {לא יצא}.
Chofetz Chaim, Silverstein translation
And know also that even if in demeaning his friend he demeaned himself with the very same slur — even if he began by railing thus against himself, he has nevertheless not left the ranks of the slanderers.