Sahih Muslim Book 009, Hadith Number 3475.
Chapter: It is forbidden to divorce the woman during her menses.
Ibn Umar (Allah be pleased with them) reported: I divorced my wife during the lifetime of Allah's Messenger (may peace be upon him) when she was in the state of menses. 'Umar (Allah be pleased with him) made a mention of it to Allah's Messenger (may peace be upon him), whereupon he said: Command him to take her back and leave her (in that state) until she is purified. Then (let her) enter the period of second menses, and when she is purified, then divorce her (finally) before having a sexual intercourse with her, or retain her (finally). That is the 'Idda (the prescribed period) which Allah commanded (to be kept in view) while divorcing the women. 'Ubaidullah reported: I said to Nafi': What became of that divorce (pronounced within 'Idda)? He said: It was as one which she counted.