Sahih Muslim Book 9, Hadith Number 3484.

Chapter: It is forbidden to divorce the woman during her menses.

Yunus b. Jubair reported: I said to Ibn ‘Umar (Allah be pleased with them): A person divorced his wife while she was in the state of menses, whereupon he said: Do you know ‘Abdullah b. Umar (Allah be pleased with them), for he divorced his wife in the state of menses. ‘Umar (Allah be pleased with him) came to Allah’s Apostle (may peace be upon him) and asked him, and he (the Holy Prophet) commanded him that he should take her back, and she started her ‘Idda. I said to him: When a person divorces his wife, and she is in the state of menses, should that pronouncement of divorce be counted? He said: Why not, was he hopeless or foolish?