Eruvin 92A

מי אמר רבי יוחנן הכי? והא אמר רבי יוחנן: הלכה כסתם משנה. ותנן: כותל שבין שתי חצירות, גבוה עשרה ורוחב ארבעה ⁠— מערבין שנים, ואין מערבין אחד. היו בראשו פירות ⁠— אלו עולין מכאן ואוכלים, ואלו עולין מכאן ואוכלים, ובלבד שלא יורידו למטה?

Did R' Yoḥanan actually say this, that the halakha is in accordance with R' Shimon’s opinion that all courtyards constitute a single domain, even if each courtyard established an independent eiruv? But didn’t R' Yoḥanan say that the halakha is in accordance with an unattributed Mishnah, and we learned: With regard to a wall between two courtyards, 10 handbreadths high and 4 handbreadths wide, they establish two eiruvin, one for each courtyard, but they do not establish one eiruv. If there was fruit atop the wall, these, the residents of one courtyard, may ascend from here and eat it, and those, the residents of the other courtyard, may ascend from there and eat it, provided that they do not take the fruit down from atop the wall to the courtyards. According to R' Yoḥanan, all the courtyards are considered a single domain. Why may they not bring the fruit down?

מאי ״למטה״⁠ — למטה לבתים. והא תני רבי חייא: ובלבד שלא יהא זה עומד במקומו ואוכל, וזה עומד במקומו ואוכל!

The Talmud answers: What is the meaning of the word down in this context? It means down to the houses; however, it is indeed permitted to bring the fruit down to the courtyards. The Talmud raises a difficulty: But didn’t R' Ḥiyya explicitly teach in a Tosefta: Provided that neither will this one stand below in his place in his courtyard and eat, nor will that one stand in his place in his courtyard and eat?

אמר ליה: וכי רבי לא שנאה, רבי חייא מנין לו?

Rav Ashi said to Ravina: No proof can be cited from this baraita of R' Ḥiyya with regard to the Mishnah. If R' Yehuda HaNasi did not explicitly teach it in this manner, from where does his student R' Ḥiyya know it? If a halakha is not taught by the Mishnah itself, it should not be distorted to have it correspond with a Tosefta.