It is narrated from Abdullah bin Umar (may Allah be pleased with them both) that he divorced his wife while she was menstruating, in the time of the Messenger of Allah (peace and blessings be upon him). Umar bin al-Khattab (may Allah be pleased with him) asked the Messenger of Allah (peace and blessings be upon him) about it, and the Messenger of Allah (peace and blessings be upon him) said: "Command him to take her back, then keep her until she becomes pure, then menstruates, then becomes pure; then, if he wishes, he may keep her after that, and if he wishes, he may divorce her before touching her. That is the waiting period for which Allah Almighty has commanded that women be divorced."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15380