Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.123A.

Section : The Forbidden Days to Fast, woman to fast while her husband is present except with his permission.

The Messenger of Allah forbade a woman to fast if her husband was present until he gave her his permission to do so. Abu Hurairah reported that the Prophet said: “A woman is not to fast [even] for one day while her husband is present except with his permission, unless it is during Ramadan.” This is related by Ahmad, al-Bukhari, and Muslim. The scholars have interpreted this prohibition as one of forbiddance, and they allow the husband to end his wife’s fasting if she fasted without his permission and he seeks his right [to sex] from her. This is also true, obviously, for days other than those of Ramadan in which case she does not need her husband’s permission. Similarly, if she fasted without his permission because he was not present, he has the right to end her fast when he returns.

If the husband is sick or incapable of intercourse, it is permissible for the woman to fast without his permission – that is, it is similar to the case of where the husband is not present.