Bava Batra 49A

אין נאמנים; ״מודעא היו דברינו״⁠ – אין נאמנין!

they are not deemed credible. And similarly, witnesses who said: Our statement that we signed was accompanied by a preemptive declaration, are not deemed credible. They cannot negate the testimony of the document that they themselves signed by claiming that there had been a preemptive declaration. Similarly, how could Rabba bar bar Ḥana’s signing of the preemptive declaration override his signing the bill of sale?

הני מילי על פה ⁠– דלא אתי על פה ומרע לשטרא, אבל בשטרא ⁠– אתי שטרא ומרע לשטרא.

The Talmud answers: That matter of witnesses not being deemed credible to nullify a document applies only when the witnesses attempt to nullify the document by means of an oral declaration, as an oral declaration cannot come and weaken a written document. But if the witnesses attempt to nullify the bill of sale by means of testimony in another document, e.g., by signing the preemptive declaration, then this preemptive document can come and weaken a written document, in this case, the bill of sale.

גופא ⁠– אמר רב נחמן: ״אמנה היו דברינו״⁠ – אין נאמנין, ״מודעא היו דברינו״⁠ – אין נאמנין.

The Talmud returns to discuss the matter itself: Rav Naḥman says that witnesses who said: Our statement that we signed was a document of trust, are not deemed credible. And similarly, witnesses who said: Our statement that we signed was accompanied by a preemptive declaration, are not deemed credible.

ומר בר רב אשי אמר: ״אמנה היו דברינו״⁠ – אין נאמנין, ״מודעא היו דברינו״⁠ – נאמנין. מאי טעמא? שזה ניתן ליכתב, וזה לא ניתן ליכתב.

And Mar bar Rav Ashi says that witnesses who said: Our statement that we signed was a statement of trust, are not deemed credible; but witnesses who said: Our statement that we signed was accompanied by a preemptive declaration, are deemed credible. What is the reason for the difference between the cases? The reason is that this document that was accompanied by a preemptive declaration may be written, as it is merely written under duress, but that document of trust may not be written, as it is a false document. Testifying that they wrote it is self-incriminating, and the witnesses are not deemed credible to incriminate themselves.