Sahih Muslim Book 009, Hadith Number 3487.
Chapter: It is forbidden to divorce the woman during her menses.
Anas b. Sirin reported that he had heard Ibn 'Umar (Allah be pleased with them) as saying. I divorced my wife while she was in the state of menses. 'Umar (Allah be pleased with him) came to Allah's Apostle (may peace be upon him) and informed him about it, whereupon he (Allah's Apostle) said: Command him to take her back and when she is pure, then divorce her. I said to Ibn 'Umar Allah be pleased with them): Did you count that pronouncement of divorce? He said: Why not? This hadith has been narrated on the authority of Shu'ba with the same chain of transmitters but with a slight variation in wording.