Ketubot 56B

וקיימא לן: מאי ״כותב״⁠ — אומר!

And we maintain on this issue: What is the meaning of: Writes? It means: Says. In order to relinquish one’s claim to produce of the produce of his wife’s property, he does not necessarily need to write this in a document; it is sufficient to say it verbally in front of witnesses. It seems, therefore, that R' Yehuda holds that a verbal stipulation is sufficient for a monetary matter of rabbinic law.

אמר אביי: לכל יש כתובה, ולא לכל יש פירות. מילתא דשכיחא ⁠— עבדו בה רבנן חיזוק, מילתא דלא שכיחא ⁠— לא עבדו בה רבנן חיזוק.

Abaye said: There is a distinction between the two cases. Every married woman has a marriage contract, but not all husbands have the right to produce, as not every woman brings property with her into the marriage. Therefore, in relation to a common matter, such as a marriage contract, the rabbis reinforced their pronouncements about it by insisting that any stipulations to change the terms must be in writing. However, with regard to an uncommon matter, such as the produce of property, the rabbis did not reinforce their pronouncements about it, and a verbal declaration is sufficient.

הרי חמרים, דשכיחי, ולא עבדו לה רבנן חיזוק!

The Talmud challenges this answer: But with regard to the halakha of donkey drivers, which is a common matter, R' Yehuda does not hold that the rabbis reinforced their pronouncements about it.

דתנן: החמרין שנכנסו לעיר, ואמר אחד מהן: שלי חדש, ושל חברי ישן. שלי אינו מתוקן, ושל חברי מתוקן אין נאמנים. רבי יהודה אומר: נאמנים.

This is as we learned in a Mishnah (Demai 4:7): In the case of donkey drivers who entered a city to sell their wares, and one of them said: My produce is from the new crop and is still moist and not as good, but my associate’s produce is from the old crop, or he said: Mine is not fit for use, i.e., tithes have not been separated, but my associate’s produce is fit for use, the drivers are not afforded credibility. There is a suspicion that they may be lying. They may have an arrangement between them where one will make this statement in one city and in the next city they will alternate, in order to appear credible, so that one will always be able to sell his wares. R' Yehuda says: They are deemed credible. This indicates that R' Yehuda holds that a verbal stipulation is sufficient even for a common monetary matter of rabbinic law.