What is the ruling on a husband having intercourse with his wife while she is observing a voluntary fast?

A woman is not permitted to observe a supererogatory fast unless her husband grants her permission, if he has a need for her during the day. The husband has the right to invalidate her supererogatory fast by intercourse if he did not grant her permission. As for her, she may break her supererogatory fast by intercourse or by other means, due to the Messenger of Allah (peace and blessings be upon him) saying: "The one observing a supererogatory fast is the master of his own self; if he wishes, he fasts, and if he wishes, he breaks his fast." It is desirable to complete a supererogatory act after commencing it, due to the Almighty's saying: "O you who have believed, obey Allah and obey the Messenger and do not invalidate your deeds."