"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." (Using translation from Nasāʾī 3396)  

أَنَّهُ طَلَّقَ امْرَأَتَهُ وَهِيَ حَائِضٌ تَطْلِيقَةً فَانْطَلَقَ عُمَرُ فَأَخْبَرَ النَّبِيَّ ﷺ بِذَلِكَ فَقَالَ لَهُ النَّبِيُّ ﷺ «مُرْ عَبْدَ اللهِ فَلْيُرَاجِعْهَا فَإِذَا اغْتَسَلَتْ فَلْيَتْرُكْهَا حَتَّى تَحِيضَ فَإِذَا اغْتَسَلَتْ مِنْ حَيْضَتِهَا الْأُخْرَى فَلَا يَمَسَّهَا حَتَّى يُطَلِّقَهَا فَإِنْ شَاءَ أَنْ يُمْسِكَهَا فَلْيُمْسِكْهَا فَإِنَّهَا الْعِدَّةُ الَّتِي أَمَرَ اللهُ أَنْ تُطَلَّقَ لَهَا النِّسَاءُ»