Sahih Muslim Book 009, Hadith Number 3485.
Chapter: It is forbidden to divorce the woman during her menses.
Ibn 'Umar (Allah be pleased with them) reported: I divorced my wife while she was in the state of menses. 'Umar (Allah he pleased wish him) came to Allah's Apostle (may peace be upon him) and made mention of that to him, whereupon Allah's Apostle (may peace be upon him) told that we should take her back, and when she is pure he may divorce her, if he would so wish. I (one of the narrators) said to Ibn 'Umar (Allah be pleased with them): Did you count (this pronouncement of divorce) in her case? He said: What (after all) prevents him from doing so? Do you find him (Ibn Umar) either helpless or foolish?