Sahih Muslim Book 009, Hadith Number 3473.
Chapter: It is forbidden to divorce the woman during her menses.
Ibn 'Umar (Allah be pleased with them) reported that he divorced his wife while she was menstruating during the lifetime of Allah's Messenger (may peace be upon him). 'Umar b. Khattib (Allah be pleased with him) asked Allah's Messenger (may peace be upon him) about it, whereupon Allah's Messenger (may peace be upon him) said: Command him ('Abdullah b. 'Umar) to take her back (and keep her) and pronounce divorce when she is purified and she again enters the period of menstruation and she is again purified (after passing the period of menses), and then if he so desires he may keep her and if he desires divorce her (finally) before touching her (without having an intercourse with her), for that is the period of waiting ('idda) which God, the Exalted and Glorious, has commanded for the divorce of women.