Nedarim, Chapter Eleven, Mishnah Nine
The Torah states that a widow and a divorcees vows are binding. This ruling is seemingly obvious, for a widow and a divorcee do not have any husband who could possibly annul their vows. In order to prevent the Torah from being obvious, the mishnah finds situations in which the ruling is not so obvious.
But every vow of a widow and of a divorcee. . . shall be binding upon her (Numbers 30:9).
1) How is this so?
a) If she said, Behold, I will be a nazirite after thirty days, even if she married within the thirty days, he cannot annul it.
2) If she vows while in her husbands domain, he can annul [the vow] for her.
a) How is this so?
b) If she said, Behold, I will be a nazirite after thirty days, [and her husband annulled it], even though she was widowed or divorced within the thirty days, it is annulled.
3) If she vowed on one day, and he divorced her on the same day and took her back on the same day, he cannot annul it.
a) This is the general rule: once she has gone into her own domain [even] for a single hour, he cannot annul.
After quoting the verse from Numbers, the mishnah proceeds to explain it in a way that it is not overly simple.
Section one: In this case the woman vowed before she was married but vowed in such a way that the vow would not begin to take effect for another thirty days. During the thirty days before the vow takes effect, she was married. Although the vow begins to take effect when she is already married, her husband cannot annul it because she made the vow before she was married.
Section two: This case is opposite to that in the previous section. Here the woman vows while still married, but stipulates that the vow will not take effect for another thirty days. While still married, before the vow takes effect, the husband annuls the vow. Even though by the time the vow took effect she was already divorced, since the husband annulled it while she was still married, the vow is annulled.
Section three: In this case, she vowed while married, was then divorced and then remarried on the same day (seemingly a theoretical situation, or two people who have trouble making up their minds!). The mishnah teaches that the husband can no longer annul the vow that she took during their first marriage, even though the second marriage was on the very same day. In the last clause, containing the general rule, the mishnah explains that if she had gone out of her husbands domain before he annulled the vow, he cannot subsequently annul the vow. This is what the Torah means when it states that a widow or divorcee must keep their vows. Sometimes they must keep them even though they made the vow when they were married.