Voluntary Fasting is with the Permission of the Husband
Question :
Do I have the right to prevent my wife from fasting days of voluntary fasting, like the six days of Shawwal? Is there any sin on me for that?
Answer:
It has been reported that it is prohibited for the woman to fast a voluntary fast while her husband is present, except with his permission. This is due to the need of seeking pleasure (i.e., by sexual relations with the wife). Hence, if the woman fasts without his permission, it is permissible for him to make her break her fast if he needs to have sexual intercourse. If he does not feel the need to have sex with her, it is disliked for him to prevent her (from fasting) if the fast will not harm her or hinder her from tending to their child or breast-feeding it and similar things. It makes no difference whether that fast is of the six days of Shawwal or other voluntary fasts.
Source:
Ash-Shaykh Ibn Jibreen
Fatawa Islamiyah, Vol. 3 Page 339