Sunan an-Nasa'i, hadith 3396

أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا الْمُعْتَمِرُ، قَالَ سَمِعْتُ عُبَيْدَ اللَّهِ بْنَ عُمَرَ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ، أَنَّهُ طَلَّقَ امْرَأَتَهُ وَهِيَ حَائِضٌ تَطْلِيقَةً فَانْطَلَقَ عُمَرُ فَأَخْبَرَ النَّبِيَّ صلى الله عليه وسلم بِذَلِكَ فَقَالَ لَهُ النَّبِيُّ صلى الله عليه وسلم ‏ "‏ مُرْ عَبْدَ اللَّهِ فَلْيُرَاجِعْهَا فَإِذَا اغْتَسَلَتْ فَلْيَتْرُكْهَا حَتَّى تَحِيضَ فَإِذَا اغْتَسَلَتْ مِنْ حَيْضَتِهَا الأُخْرَى فَلاَ يَمَسَّهَا حَتَّى يُطَلِّقَهَا فَإِنْ شَاءَ أَنْ يُمْسِكَهَا فَلْيُمْسِكْهَا فَإِنَّهَا الْعِدَّةُ الَّتِي أَمَرَ اللَّهُ عَزَّ وَجَلَّ أَنْ تُطَلَّقَ لَهَا النِّسَاءُ ‏"‏ ‏.‏

It was narrated from 'Abdullah that he issued a divorce to his wife when she was menstruating. So 'Umar went to inform the Prophet about that. The Prophet said to him:"Tell 'Abdullah to take her back, then, when she has performed Ghusl, let him leave her alone, until she menstruates (again). Then, when she performs Ghusl following that second period, he should not touch her until he divorces her. And if he wants to keep her, then let him keep her. That is the time when Allah has stated that women may be divorced