Sahih Muslim Book 009, Hadith Number 3474.
Chapter: It is forbidden to divorce the woman during her menses.
Abdullah (b. 'Umar) reported that he divorced a wife of his with the pronouncement of one divorce during the period of menstruation. Allah's Messenger (may peace be upon him) commanded him to take her back and keep her until she was purified, and then she entered the period of menses in his (house) for the second time. And he should wait until she was purified of her menses. And then if he would decide to divorce her, he should do so when she was purified before having a sexual intercourse with her; for that was the 'Idda which Allah had commanded for the divorce of women. Ibn Rumh in his narration made this addition: When 'Abdullah was asked about it, he said to one of them: If you have divorced your wife with one pronouncement or two (then you can take her back), for Allah's Messenger (may peace be upon him) commanded me to do it; but if you have divorced her with three pronouncements, then she is forbidden for you until she married another husband, and you disobeyed Allah in regard to the divorce of your wife what He had commanded you. (Muslim said: The word "one divorce" used by laith is good.)