Is it permissible for a woman to fast voluntarily without her husband's permission, knowing that he is away from home from dawn until sunset or an hour before that?

A woman is not permitted to observe a voluntary fast while her husband is present unless she has his permission. This is based on the statement of the Prophet, peace and blessings be upon him: "No woman should fast while her husband is present except with his permission." The reason for this prohibition is that the husband has the right to enjoy his wife at any time, and his right is immediately due and should not be forfeited by a voluntary fast. However, if the husband is traveling, she is permitted to observe a voluntary fast because he is unable to enjoy her.