Sahih Muslim
The Book of Divorce (Kitab Al-Talaq)
Book Number: 009 / Hadith Number: 3480
Ibn 'Umar (Allah be pleased with them) reported that he divorced his wife while she was in her menses. 'Umar (Allah be pleased with him) asked Allah's Apostle (may peace be upon him) about that, and he said: Command him to take her back until she is pure and then she enters the second menses and then becomes pure. Then either divorce her (finally) or retain her.