Sahih Muslim Book 009, Hadith Number 3479.

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 in the state of menses. 'Umar (Allah be pleased with him) made mention of it to Allah's Apostle (may peace be upon him) and he said: Command him to take her back, then divorce her when she is pure or she is pregnant.