Quote:
The prescribed period at which Allaah has commanded men to divorce women means that a man may only divorce his wife when she is pure (not menstruating) and he has not had intercourse with her. Based on this, if he divorces her when she is menstruating, he has not divorced her in accordance with the command of Allaah, so it is to be rejected. We think that the divorce that has been issued to this woman does not count as such, and that the woman is still married to her husband. It does not matter whether the husband knew that she was menstruating or not menstruating when he issued the divorce. Yes, his knowledge of her condition does not matter, but if he knew about that then he has committed a sin, and the divorce does not count. If he did not know, then the divorce still does not count, but there is no sin on the husband.
It is always best though to consult your own local imam for clarity and guidance.