Salim narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that he divorced his wife while she was menstruating, and Sayyiduna Umar (may Allah be pleased with him) told the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) said: "Order him to take her back, then to divorce her when she is pure or pregnant."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 14934