Gittin 27B

בזמן שהבעל מודה מיהת ⁠– יחזיר לאשה, ואפילו לזמן מרובה! ומשני: כאן במקום שהשיירות מצויות, כאן במקום שאין השיירות מצויות.

when the husband admits that he wrote it, in any event he should return it to the woman, and by omission this appears to be the halakha even if a long time has passed since the bill of divorce was lost. And R' Zeira answers: Here, in the Mishnah where it states that he should not return it, it is stated with regard to a place where caravans are frequently found; there, in the baraita where it states that he should return it, it is stated with regard to a place where caravans are not frequently found.

איכא דאמרי והוא שהוחזקו הוא דלא ליהדר, והיינו דרבה; ואיכא דאמרי אף על גב דלא הוחזקו לא ליהדר, ופליגא דרבה.

The Talmud compares the rulings of Rabba and R' Zeira. There are those who say with regard to R' Zeira’s statement that he should not return it in a place where caravans are frequently found: And this is the case when it is established that there are two people in the town with the identical name. In that case, R' Zeira holds that it should not be returned, and this is the same ruling as that of Rabba. And there are those who say: In a place where caravans are frequently found, even though it is not established that there are two people with identical names, it should not be returned, and he disagrees with the ruling of Rabba.

בשלמא דרבה לא אמר כרבי זירא, מתניתין אלימא ליה לאקשויי; אלא רבי זירא, מאי טעמא לא אמר כרבה?

The Talmud clarifies: Granted that Rabba did not say a discourse like that of R' Zeira and raise a contradiction from the baraita, as he holds that it is a stronger challenge to raise a difficulty from the Mishnah in tractate Bava Metzia because the Mishnah, redacted by R' Yehuda HaNasi, employs more precise language than baraitot. But what is the reason that R' Zeira did not say a discourse like that of Rabba and raise a contradiction from the Mishnah in tractate Bava Metzia?