Short answer
Not a voluntary fast while her husband is at home, unless he agrees. The Prophet ﷺ said a woman should not fast while her husband is present except with his permission. This applies to nafl fasts only. Ramadan needs no permission, and neither does making up Ramadan when time is short. If he is travelling, she may fast freely.
A wife may not begin a nafl or open vowed fast without her husband's consent while he is resident, as it may stand between him and his right; it is makruh tahrimi, and he may ask her to break it, in which case she makes it up later. Ramadan needs no permission; its qada is also an obligation, though while there is time to spare she should choose days that suit him.
The four schools agree on this.
Explanation
Abu Hurayrah reported that the Prophet ﷺ said: it is not lawful for a woman to fast while her husband is present except with his permission. The scholars explain that the wording refers to voluntary fasts, because a husband has rights of intimacy and companionship that a daylong fast affects, and a voluntary act cannot override an obligation.
What needs permission:
- Sunnah and nafl fasts: Mondays, Thursdays, the white days, Shawwal, Arafah, Ashura.
- A vow that was not tied to a fixed date.
What does not:
- Ramadan itself.
- Making up Ramadan when only enough days remain before the next Ramadan. Earlier in the year she should choose times that suit both of them, as Aisha did when she made up her fasts in Sha'ban.
- Any fast while the husband is away travelling.
Kindness on both sides. A general standing permission is enough: she does not need to ask each time. A husband should not refuse without good reason, since he would be keeping her from a great reward, and many husbands are happy to fast together. If he asks her to break a voluntary fast, she does so and makes it up later. The four schools agree on the principle. See rights of husband and wife and the sunnah fasts.
The evidence
Sahih al-Bukhari 5192
Sahih... لا تصوم المرأة وبعلها شاهد إلا بإذنه ...
"A woman should not fast while her husband is present, except with his permission."
A wife's voluntary fast needs her husband's agreement when he is at home.
Where the ruling is recorded
- al-Durr al-Mukhtar, Kitab al-Sawm, chapter on what breaks the fast. A wife does not fast voluntarily without her husband's permission, and he may have her break it, with qada later.
- al-Fatawa al-Hindiyya, Kitab al-Sawm, chapter on what is desirable and disliked in fasting. The voluntary fast of a wife without permission is disliked.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.