Ketubot 13A

דאם כן, קשיא הלכתא אהלכתא. דקיימא לן הלכתא כרב נחמן בדיני, ובהא אמר רב יהודה אמר שמואל: הלכה כרבן גמליאל. אלא לאו שמע מינה כדמשנינן. שמע מינה.

Because if that were not so, it is difficult, as there would be a contradiction between one halakha and another halakha. Since, on the one hand, we maintain a principle in halakhic ruling that the halakha is in accordance with the opinion of Rav Naḥman in civil law; and, on the other hand, in that case involving the claims of the bride and the groom Rav Yehuda said that Shmuel said: The halakha is in accordance with the opinion of Rabban Gamliel. Rather, can we not conclude from this apparent contradiction that the matter must be understood as we are teaching, that the opinion of Rav Naḥman can be reconciled with the opinion of Rabban Gamliel? The Talmud affirms: Indeed, conclude from it that this is the case.

מתני׳ היא אומרת: מוכת עץ אני. והוא אומר: לא כי, אלא דרוסת איש את. רבן גמליאל ורבי אליעזר אומרים: נאמנת. ורבי יהושע אומר: לא מפיה אנו חיין, אלא הרי זו בחזקת דרוסת איש, עד שתביא ראיה לדבריה.

Mishnah: In a case where she says: I am one whose hymen was ruptured by wood, i.e., she admits that her hymen is not intact but claims that it was not ruptured through sex, and the groom says: No; rather, you are one who was trampled by a man, and your hymen was ruptured through sex, Rabban Gamliel and R' Eliezer say: She is deemed credible and her claim is accepted because she certainly knows what actually happened. R' Yehoshua says: It is not based on the statement emerging from her mouth that we conduct our lives; rather, she retains the presumptive status of one who was trampled by a man, until she brings proof supporting her statement that her hymen was ruptured by wood.