Sahih Muslim Book 009, Hadith Number 3486.
Chapter: It is forbidden to divorce the woman during her menses.
Anas b. Sirin reported: I asked Ibn 'Umar (Allah be pleased with them) about the woman whom he had divorced. He said: I divorced her while she was in the state of menses. It was mentioned to 'Umar (Allah be pleased with him) and he then made a mention of that to Allah's Apostle (may peace be upon him), whereupon he said: Command him to take her back and when the period of menses is over, then (he may divorce her in the state of her purity. He (Ibn Umar) said: So I took her back, then divorced her in her purity. I (the narrator) said: Did you count that divorce which you pronounced in the state of menses? He said: Why should I not have counted that? Was I helpless or foolish?