Nafi' 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) mentioned this to the Prophet (peace and blessings be upon him). He (peace and blessings be upon him) counted it as one divorce.
(b) Sa'id bin Jubayr narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that he said: It was counted against me as one divorce.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 14928