Nafi' narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that he divorced his wife while she was menstruating. Sayyiduna Umar (may Allah be pleased with him) asked the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) ordered him to take her back, and said: "Then let him give her time until another menstruation, then give her time until she becomes pure, and then divorce her before having intercourse." And he said: "This is the time for which Allah has commanded that women be divorced." Whenever Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) was asked about a divorce given to a woman while she was menstruating, he would say: "If you gave one or two divorces, the Messenger of Allah (peace and blessings be upon him) ordered him to take her back, then to give her time until another menstruation, and then divorce (her) in purity without having had intercourse. But if you gave three divorces, then you have disobeyed the command Allah gave you concerning women, and the woman has become separated from you."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرجعة / 15152