Sunan an-Nasai Book 27, Hadith 3390

أَخْبَرَنَا مُحَمَّدُ بْنُ سَلَمَةَ، قَالَ أَنْبَأَنَا ابْنُ الْقَاسِمِ، عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّهُ طَلَّقَ امْرَأَتَهُ وَهِيَ حَائِضٌ فِي عَهْدِ رَسُولِ اللَّهِ صلى الله عليه وسلم فَسَأَلَ عُمَرُ بْنُ الْخَطَّابِ رضى الله عنه رَسُولَ اللَّهِ صلى الله عليه وسلم عَنْ ذَلِكَ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ مُرْهُ فَلْيُرَاجِعْهَا ثُمَّ لْيُمْسِكْهَا حَتَّى تَطْهُرَ ثُمَّ تَحِيضَ ثُمَّ تَطْهُرَ ثُمَّ إِنْ شَاءَ أَمْسَكَ بَعْدُ وَإِنْ شَاءَ طَلَّقَ قَبْلَ أَنْ يَمَسَّ فَتِلْكَ الْعِدَّةُ الَّتِي أَمَرَ اللَّهُ عَزَّ وَجَلَّ أَنْ تُطَلَّقَ لَهَا النِّسَاءُ ‏"‏ ‏.‏

It was narrated from Ibn 'Umar that he divorced his wife while she was menstruating, during the time of the Messenger of Allah. 'Umar bin Al-Khattab, may Allah be pleased with him, asked the Messenger of Allah about that, and the Messenger of Allah said:"Tell him to take her back and keep her until she becomes pure, then menstruates again and becomes pure again. Then if he wishes he may keep her, or if he wishes, he may divorce her before he touches (has intercourse with) her. This is the time when Allah, the Mighty and Sublime, has stated that women may be divorced