Eruvin 13A

רבי עקיבא אומר: על זה ועל זה נחלקו כו׳.

The Mishnah relates that a student recited a halakha before R' Akiva, and he did not accept the student’s version of the dispute between Beit Shammai and Beit Hillel, as R' Akiva said: They disagree about this, an alleyway less than 4 cubits wide, and about that, an alleyway more than 4 cubits wide.

רבי עקיבא היינו תנא קמא! איכא בינייהו דרב אחלי, ואיתימא רב יחיאל. ולא מסיימי.

The Talmud asks: In that case, the opinion of R' Akiva is identical with the opinion first tanna of the Mishnah, as he too holds that Beit Shammai and Beit Hillel disagree in all cases, irrespective of the width of the alleyway. The Talmud answers: There is a practical difference between them with regard to the halakha stated by Rav Aḥlai, and some say it was Rav Yeḥiel, that an alleyway less than 4 handbreadths wide requires no corrective action. However, their respective opinions are not defined; which tanna accepts the view of Rav Aḥlai and which tanna rejects it cannot be determined.

תניא, אמר רבי עקיבא: לא אמר רבי ישמעאל דבר זה, אלא אותו תלמיד אמר דבר זה, והלכה כאותו תלמיד.

It was taught in a baraita that R' Akiva said: R' Yishmael did not state this matter, as it is unlikely that R' Yishmael would err in this manner; rather, it was that disciple who stated that matter on his own, and the halakha is in accordance with the opinion of that disciple.

הא גופה קשיא: אמרת ״לא אמר רבי ישמעאל דבר זה״⁠ — אלמא לית הלכתא כוותיה, והדר אמרת: ״הלכה כאותו תלמיד״!

With regard to that baraita the Talmud asks: This baraita itself is difficult. You stated initially that R' Yishmael did not state this matter; apparently the halakha is not in accordance with the opinion of the disciple. And then you said: The halakha is in accordance with the opinion of that disciple.