Is it permissible for a woman to observe a voluntary fast without her husband's knowledge, given her certainty that he would not object, her desire for the fast to be among her hidden good deeds, and her commitment to breaking the fast should her husband request his marital right?

The basic ruling concerning a wife's voluntary fasting is that it is forbidden unless her husband gives permission, because it infringes upon his right. The proof for this is the Prophet's (peace and blessings be upon him) saying: "A woman should not fast while her husband is present except with his permission."

However, if the wife knows that her husband would not prevent her from voluntary fasting, then it is permissible for her to fast without his permission. In this case, there is no sin upon her.