Bava Batra 170A

רבן שמעון בן גמליאל סבר: אין אותיות נקנות במסירה, ורבי סבר: אותיות נקנות במסירה.

The Talmud explains: This is a case where one party sold land to another, who then sold it to a third party by transferring the bill of sale to him. Rabban Shimon ben Gamliel holds that the letters are not acquired by transferring possession of the document. Handing the bill to the third party is therefore not a valid act of acquisition. In order to establish that the land is his, one must prove presumptive ownership by demonstrating that he has lived in the land uncontested for 3 years. And R' Yehuda HaNasi holds that the letters are acquired by transferring possession of the document. Therefore, if the third party produces the bill of sale that had been transferred to him, this is sufficient proof that the land is his.

אמר ליה אביי: אם כן, פלוגתא לדמר! אמר ליה: ותפלוג!

Abaye objected and said to Rav Dimi: If that is so, that Rabban Shimon ben Gamliel holds that letters are not acquired by transferring possession of the bill of sale, this would be in disagreement with what the Master, Rabba, said earlier, that Rabban Shimon ben Gamliel maintains that letters are acquired by transferring possession of the bill of sale. Rav Dimi said back to him: And let it be in disagreement. I am not obligated to be in agreement with Rabba.

אמר ליה, הכי קאמינא לך: מתניתא לא מיתרצא אלא כדמתרצא מר, ואם כן קשיא דרבן שמעון בן גמליאל אדרבן שמעון בן גמליאל!

Abaye clarified his objection and said to him: This is what I meant to say to you. The baraita (168b), which Rabba was explaining, can be explained only the way the Master, Rabba, explained it. And if so, i.e., if you do not agree with Rabba’s understanding of the opinion of Rabban Shimon ben Gamliel, there is a difficulty, as there is a contradiction between one statement of Rabban Shimon ben Gamliel and the other statement of Rabban Shimon ben Gamliel.