Sunan an-Nasa'i 3389

Nafi' narrated from 'Abdullah, that he divorced his wife while she was menstruating. 'Umar asked the Messenger of Allah about that and said:"Abdullah has divorced his wife while she was menstruating." He said: "Tell 'Abdullah to take her back, then leave her until she becomes pure from this menstrual period, then menstruates again, then when she becomes pure again, if he wishes he may separate from her before having intercourse with her, or if he wishes he may keep her. This is the time when Allah, the Mighty and Sublime, has stated that women may be divorced