daf bit: Nedarim 88

The torah teaches that a husband can annul his wife's vow and a father can annul his unmarried daughter's vow, but only on the day they hear of it. The mishna on today's daf teaches that every vow of a widow or a divorced woman stands. Now, what about some timing questions? If she vowed and her husband annulled and then he divorced her the vow is annulled, but if he had already divorced her the vow stands. If she vowed and he divorced her and took her back all on the same day, the vow stands. This is the general rule: once she has gone forth as her own mistress even for a single hour, he cannot annul. (88b-89a)