Yom Kippur - Kol Nidre

(3) If a man makes a vow to the LORD or takes an oath imposing an obligation on himself, he shall not break his pledge; he must carry out all that has crossed his lips.

(3) When you make a vow to God, do not delay to fulfill it. For God has no pleasure in fools; what you vow, fulfill. (4) It is better not to vow at all than to vow and not fulfill. (5) Don’t let your mouth bring you into disfavor, and don’t plead before the messenger that it was an error, but fear God; else God may be angered by your talk and destroy your possessions.

Rava said to Rav Naḥman: Master, see that Sage who came from the West, Eretz Yisrael, and who said: The Sages attended to the dissolution of a vow taken by the son of Rav Huna bar Avin, and they dissolved his vow and said to him: Go and request mercy for yourself, for you have sinned by taking a vow. As Rav Dimi, the brother of Rav Safra teaches: With regard to anyone who takes a vow, even if he fulfills it, he is called a sinner. Rav Zevid said: What verse teaches this? It is: “But if you refrain to vow, it will be no sin in you” (Deuteronomy 23:23). It may be inferred that if you did not refrain from taking vows, there is sin.

The Gemara relates that Rav Huna bar Ḥinnana intended to teach this topic at the Festival lecture, so that everyone would learn this manner of rendering vows void on Rosh HaShana. Rava said to him: The tanna of the mishna conceals it and does not say it explicitly, despite the fact that it is studied by Torah scholars, in order that the public not treat vows lightly, and you teach it publicly at the Festival lecture?