Musnad Ahmad 27:1342

Muhammad ibn Ubayd narrated to us, Ubaydullah narrated to us, from Nafi’, from Ibn Umar, who said: I divorced my wife during the time of the Messenger of Allah (peace and blessings of Allah be upon him) while she was menstruating. Umar mentioned that to the Messenger of Allah (peace and blessings of Allah be upon him), and he said: "Command him to take her back until she becomes pure, then she menstruates another time, then when she becomes pure, he may divorce her if he wishes before he has intercourse with her, or he may keep her. For that is the waiting period which Allah has commanded that women be divorced for."