Musar for Pirkei Avot 3:3
רַבִּי שִׁמְעוֹן אוֹמֵר, שְׁלשָׁה שֶׁאָכְלוּ עַל שֻׁלְחָן אֶחָד וְלֹא אָמְרוּ עָלָיו דִּבְרֵי תוֹרָה, כְּאִלּוּ אָכְלוּ מִזִּבְחֵי מֵתִים, שֶׁנֶּאֱמַר (ישעיה כח) כִּי כָּל שֻׁלְחָנוֹת מָלְאוּ קִיא צֹאָה בְּלִי מָקוֹם. אֲבָל שְׁלשָׁה שֶׁאָכְלוּ עַל שֻׁלְחָן אֶחָד וְאָמְרוּ עָלָיו דִּבְרֵי תוֹרָה, כְּאִלּוּ אָכְלוּ מִשֻּׁלְחָנוֹ שֶׁל מָקוֹם בָּרוּךְ הוּא, שֶׁנֶּאֱמַר (יחזקאל מא) וַיְדַבֵּר אֵלַי זֶה הַשֻּׁלְחָן אֲשֶׁר לִפְנֵי ה':
R. Shimon says: If three ate at one table and did not speak words of Torah at it, it is as if they ate of the offerings of the dead [i.e., offerings to idolatry, viz. (Psalms 106:28): "They cleaved to Baal Peor and they ate the offerings of the dead"], as it is written (Isaiah 28:8): "For all the tables are full of vomit, excrement, [and idolatry is called excrement (tzoah), viz. (Isaiah 30:22): "You will tell it (idolatry) Be gone! (Tze)] without makom" [i.e., (homiletically:) because they did not mention the name of Makom (the L rd) at the table. But by the grace at the table the obligation is fulfilled and it is as if they spoke words of Torah at it (Thus have I heard).] But if three ate at one table and spoke words of Torah at it, it is as if they ate from the table of the Makom, blessed is He, as it is written (Ezekiel 41:22): "And He said to me: 'This is the table that is before the L rd.'" (Some say that this is derived from the beginning of the verse, viz.: "And the altar was of wood, three amoth" — do not read it "amoth," but "eimoth," as in "Yesh em lemikrah" ("There is support in the reading"). "Three" (supports) — Torah, Prophets, and Writings; others say: Scripture, Mishnah and Talmud, in which a man must converse at the table, whereupon it is called "the table that is before the L rd." (Thus, Rashi).]