Is the husband sinful if he prevents his sick wife from fasting 'Ashura' out of fear that she will discontinue her treatment, knowing that she will not be negatively affected if she does not take her medicine, and is she sinful if she disobeys him and fasts?

A woman is not permitted to observe a supererogatory fast without the permission of her present husband, based on the Prophet, peace and blessings be upon him, who said: "No woman should fast while her husband is present except with his permission." The husband has the right to prevent his wife from observing a voluntary fast, and there is no sin upon him for doing so. If the wife fasts without her husband's permission, her fast is valid, though she incurs sin. The Shafi'is hold that she does not incur sin for fasting on non-recurring days like Ashura and Arafat, unless her husband explicitly forbids her.