Musnad Abu Ya'la al-Mawsili 2:52

Al-Qawariri narrated to us, Bishr bin al-Mufaddal and Yahya bin Sa'id narrated to us, from Ubaydullah bin Umar, from Nafi', that Ibn Umar narrated to him that he divorced his wife with one pronouncement while she was menstruating. Umar asked the Messenger of Allah (peace be upon him) for a legal ruling. He said: "Command Abdullah bin Umar to take her back, then let him keep her until she is pure from this menstruation of hers. Then, if she menstruates another time and becomes pure, if he wishes, he may divorce her before he has intercourse with her, and if he wishes, he may keep her." For that is the waiting period (Iddah) for which Allah has commanded that women be divorced.