Amir says that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) gave his wife one divorce while she was menstruating, so Sayyiduna Umar (may Allah be pleased with him) informed the Messenger of Allah (peace and blessings be upon him). He (peace and blessings be upon him) ordered: "When she becomes pure, let him take her back, then divorce her within her waiting period, and the divorce he gave the first time is to be counted as one." [Hasan]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 14926