Pesachim 55A

תנא קולי קולי קתני.

The tanna is teaching a series of leniencies. He taught only those aspects in which the 9th of Av is more lenient than a communal fast. He did not teach those aspects in which it is more stringent. There was no attempt made to enumerate all the differences.

ובכל מקום תלמידי חכמים וכו׳. למימרא דרבן שמעון בן גמליאל סבר: לא חיישינן ליוהרא, ורבנן סברי: חיישינן ליוהרא? והא איפכא שמעינן להו, דתנן: חתן, אם ירצה לקרות קרית שמע לילה הראשון ⁠— קורא. רבן שמעון בן גמליאל אמר: לא כל הרוצה ליטול את השם יטול.

It was stated in the Mishnah: And in all places Torah scholars are idle and do not perform labor on the 9th of Av, and according to Rabban Shimon ben Gamliel one should always conduct himself like a Torah scholar in this regard and refrain from performing labor. The Talmud asks: Is that to say that Rabban Shimon ben Gamliel holds that we are not concerned about presumptuousness when a person conducts himself like a Torah scholar? And conversely, do the Rabbis hold that we are concerned about presumptuousness? Didn’t we hear them say the opposite? As we learned in a Mishnah: With regard to the recitation of Shema on one’s wedding night, the Rabbis said that if a groom wishes to recite Shema on the first night despite his exemption, he may do so. Rabban Shimon ben Gamliel says: Not everyone who wishes to assume the reputation of a God-fearing person may assume it, and consequently not everyone who wishes to recite Shema on his wedding night may do so. Their opinions in that Mishnah appear contrary to their opinions in the current Mishnah.

אמר רבי יוחנן: מוחלפת השיטה. רב שישא בריה דרב אידי אמר לא תיפוך: דרבנן אדרבנן לא קשיא; הכא, כיון דכולי עלמא עבדי מלאכה ואיהו לא עביד ⁠— מיחזי כיוהרא, אבל התם, כיון דכולי עלמא קרי ואיהו נמי קרי ⁠— לא מיחזי כיוהרא.

R' Yoḥanan said: The attribution of the opinions is reversed in one of the sources. Rav Sheisha, son of Rav Idi, said: Do not reverse either text, as it is possible to resolve the difficulty in another manner. The contradiction between the statement of the Rabbis here and the statement of the Rabbis there is not difficult. Here, on the 9th of Av, since everyone is performing labor and he is not performing labor, his idleness is conspicuous and appears like presumptuousness. However, there, in the case of reciting Shema on one’s wedding night, it does not appear like presumptuousness, as everyone is reciting Shema and he is also reciting it with them.