Chafetz Chaim, Part One, The Prohibition Against Lashon Hara, Principle 8
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וְדַע עוֹד, דַּאֲפִלּוּ (כב) לְבַזּוֹת וּלְחָרֵף אֶת הַמֵּתִים גַּם כֵּן אָסוּר, וְכָתְבוּ הַפּוֹסְקִים, דְּיֵשׁ תַּקָּנָה וְחֵרֶם קַדְמוֹנִים, שֶׁלֹּא לְהוֹצִּיא לַעַז וְשֵׁם רָע עַל הַמֵּתִים. וְכָל זֶה אֲפִלּוּ אִם הַמֵּת עַם הָאָרֶץ, וְכָל שֶׁכֵּן אִם הוּא תַּלְמִיד חָכָם, בְּוַדַּאי הַמְבַזֶּה אוֹתּוֹ עֲוֹן פְּלִילִי הוּא, וְחַיָּב נִדּוּי עַל זֶה, כְּמוֹ שֶׁנִּפְסַק בְּיוֹרֶה דֵּעָה בְּסִימָן רמ"ג סָעִיף ז'. וְאִסוּר שֶׁל הַמְבַזֶּה תַּלְמִיד חָכָם הוּא אֲפִלּוּ אִם מְבַזֶּה אוֹתוֹ בְּעַצְּמוֹ, (כג) וְכָל שֶׁכֵּן שֶׁאָסוּר לְבַזּוֹת אֶת דִּבְרֵי תּוֹרָתוֹ.
And know further, that even to shame and insult the dead is forbidden. And the poskim have written that there is a prohibition and a ban of the early masters not to defame or to spread an evil report about the dead. All this, even if the dead one was an ignoramus; how much more so, if he were a Torah scholar, is the one who shames him guilty of a great sin and liable to the ban for this! (viz. Yoreh Deah 243:7). And the issur of shaming a Torah scholar obtains even if he shames the scholar himself; how much more so is it forbidden to shame his words of Torah!