I think it is in seder moed, where a tanna or amora learns while sitting on his hands and possibly has a disputation with a min? Anyone remember this Gemara?
Shabbos 88A at the bottom of the page
הָהוּא מִינָא דְּחַזְיֵיהּ לְרָבָא דְּקָא מְעַיֵּין בִּשְׁמַעְתָּא, וְיָתְבָה אֶצְבְּעָתָא דִידֵיהּ תּוּתֵי כַּרְעָא וְקָא מָיֵיץ בְּהוּ, וְקָא מַבְּעָן אֶצְבְּעָתֵיהּ דְּמָא. אֲמַר לֵיהּ: עַמָּא פְּזִיזָא, דְּקַדְּמִיתוּ פּוּמַּיְיכוּ לְאוּדְנַיְיכוּ, אַכַּתִּי בְּפַחְזוּתַיְיכוּ קָיְימִיתוּ. בְּרֵישָׁא אִיבַּעְיָא לְכוּ לְמִשְׁמַע, אִי מָצִיתוּ — קַבְּלִיתוּ, וְאִי לָא — לָא קַבְּלִיתוּ. אֲמַר לֵיהּ: אֲנַן
The Gemara relates that a heretic saw that Rava was immersed in studying halakha, and his fingers were beneath his leg and he was squeezing them, and his fingers were spurting blood. Rava did not notice that he was bleeding because he was engrossed in study. The heretic said to Rava: You impulsive nation, who accorded precedence to your mouths over your ears. You still bear your impulsiveness, as you act without thinking. You should listen first. Then, if you are capable of fulfilling the commands, accept them. And if not, do not accept them. He said to him: About us,