Nedarim 53A

מתני׳ הנודר מן התמרים ⁠— מותר בדבש תמרים. מסתוניות ⁠— מותר בחומץ סתוניות. רבי יהודה בן בתירא אומר: כל ששם תולדתו קרויה עליו, ונודר הימנו ⁠— אסור ביוצא הימנו, וחכמים מתירים.

Mishnah: One who vows that dates are forbidden to him is permitted to eat date honey. One who vows that late grapes are forbidden to him is permitted to eat vinegar of late grapes. R' Yehuda ben Beteira says: In the case of any food that the name of its derivative is called after its name, i.e., the liquid that emerges from it bears its name, e.g., date honey or vinegar of late grapes, and one vows that the item itself, e.g., the grape, is forbidden to him, he is also prohibited from consuming the liquid that emerges from it. But the Rabbis permit this.

גמ׳ [חכמים] היינו תנא קמא?

Talmud: The statement of the Rabbis is identical to the statement of the first tanna of the Mishnah, who rules that one who vows that dates are forbidden to him is permitted to eat date honey. What is the difference between them?

איכא בינייהו הדא דתניא: כלל זה אמר רבי שמעון בן אלעזר: כל שדרכו לאכול, ודרך היוצא ממנו לאכול. כגון תמרים ודבש תמרים, נדר בו ⁠— אסור ביוצא ממנו, נודר מיוצא ממנו ⁠— אסור בו.

The Talmud answers: There is a difference between them with regard to that which is taught in a baraita: R' Shimon ben Elazar said this principle: With regard to anything that is commonly eaten in its existing form, and it is also common to eat the liquid that emerges from it, For example: dates and date honey, if one vowed that it is forbidden to him, he is also prohibited from consuming the liquid that emerges from it. Similarly, if one vows that the liquid that emerges from it is forbidden to him, he is also prohibited from partaking of it.

כל שאין דרכו לאכול, ודרך היוצא ממנו לאכול, נודר בו ⁠— אין אסור אלא ביוצא ממנו, שלא נתכוון זה אלא ליוצא ממנו.

The baraita continues: Conversely, with regard to anything that is not commonly eaten as it is, and it is common to eat the liquid that emerges from it, if one vowed that he will not eat from it, he is prohibited from consuming only the liquid that emerges from it, as this person intended to prohibit himself from eating only from the liquid that emerges from it. The first tanna does not distinguish between dates, which are commonly eaten in their original state, and late grapes, which are not. In both cases, he rules that the produce itself is forbidden and the derivative is permitted. By contrast, the Rabbis agree with R' Shimon ben Elazar’s ruling with regard to produce that is not commonly eaten in its original state. They permit the produce itself and prohibit that which has emerged from it. Therefore, in the last case of the Mishnah, they permit eating the late grapes and prohibit eating their vinegar, whereas in the first case they prohibit both dates and date honey.