Reference: Fataawa Ramadhaan – Volume 2, Page 673, Fatwa No.676
Fataawa as-Siyaam – Page 96
Question: Do I have the right to prevent my wife from fasting voluntary fasts such as the six [fasts] of Shawwaal? And would I be liable to sin if I did that?
Response: It has been mentioned that it is not allowed for a woman to fast voluntary fasts in the presence of her husband (i.e. when her husband is at home and not away on travel) except with his permission, since he may wish to seek pleasure [with his wife by having sexual relations]. So if she fasted without his permission, then it is permissible for him to ask her to break it if he wishes to have sexual intercourse with her.
If, however, he has no [sexual] needs to satisfy with her, then it is disliked for him to prevent her from fasting so long as the fast is not affecting her and nor is it hindering her from tending to the children or breast feeding or the like; This is the case if she was fasting the six [fasts] of Shawwaal or other than them from the voluntary fasts.