Nafi' narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with him) that he said: I divorced my wife in the time of the Prophet (peace and blessings be upon him) while she was menstruating, and Sayyiduna Umar (may Allah be pleased with him) mentioned it before the Prophet (peace and blessings be upon him). The Messenger of Allah (peace and blessings be upon him) said: "Command him to take her back until she becomes pure, then she has another menstruation; when she becomes pure, then if he wishes he may divorce her before having intercourse with her, or keep her. This is the waiting period in which Allah, the Lord of Might, has commanded that women be divorced." I said to Nafi': "What became of that divorce?" He said: "It was one (divorce), and it was counted."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 14907