Nedarim 60B

אמר ליה: אי הכי, ליגזור ב״יום אחד״ משום ״היום״!

Abaye said to him: If so, the rabbis should likewise decree in the case of one who takes a vow for one day that he must keep the vow until nightfall of the following day, due to the confusion that might be caused in a case where one said that his vow applies today. If the vow expires in the middle of the day, 24 hours after he took the vow, people might think that if one takes a vow in the morning and applies it to this day, it also expires in the middle of the day.

אמר ליה: ״היום״ ב״יום אחד״ מיחלף, ״יום אחד״ ב״היום״ לא מיחלף.

Rav Yosef said to him: A vow taken for this day might be interchanged with a vow taken for one day, and one might erroneously conclude that a vow taken for one day expires at nightfall. However, a vow taken for one day is not interchanged with a vow taken for today, and there is no concern that one who takes a vow for today will erroneously conclude that it expires in the middle of the day.

אמר רבינא: אמר לי מרימר, הכי אמר אבוך משמיה דרב [יוסף]: כמאן אזלא שמעתיה דרב ירמיה בר אבא ⁠— כרבי נתן. דתניא, רבי נתן אומר: כל הנודר ⁠— כאילו בנה במה, והמקיימו ⁠— כאילו מקטיר עליה.

Ravina said: Mareimar said to me: Your father said as follows, in the name of Rav Yosef: In accordance with whose opinion is this halakha taught by Rav Yirmeya bar Abba? It is in accordance with the opinion of R' Natan, as it is taught in a baraita: R' Natan says: Anyone who vows, it is as if he has built a personal altar, which is forbidden because one must bring all offerings to the Temple. And one who fulfills the vow, is as though he burns portions meant for the altar in the Temple upon it, i.e., the personal altar, thereby increasing his sin. Consequently, even after he has fulfilled the vow, it is preferable for him to ask a halakhic authority to annul it entirely, so that it will be as if he never took a vow.