"I asked Ibn Umar about a man who divorced his wife while she was menstruating. So he said: 'Don't you know Abdullah bin Umar?' Indeed he divorced his wife while she was menstruating, so Umar asked the Prophet about that, and he ordered him to take her back.'" He said: "I said: And that divorce is counted? He said: And that divorce is counted? He said: 'What else would you think if he was helpless and foolish?'"  

هَلْ تَعْرِفُ عَبْدَ اللَّهِ بْنَ عُمَرَ فَإِنَّهُ طَلَّقَ امْرَأَتَهُ وَهِيَ حَائِضٌ فَسَأَلَ عُمَرُ النَّبِيَّ ﷺ فَأَمَرَهُ أَنْ يُرَاجِعَهَا قَالَ قُلْتُ فَيُعْتَدُّ بِتِلْكَ التَّطْلِيقَةِ قَالَ فَمَهْ أَرَأَيْتَ إِنْ عَجَزَ وَاسْتَحْمَقَ  

His father divorced his wife during her menses, so Umar asked the Prophet about that and he said: "Tell him to take her back, then let him divorce her while she is pure or pregnant."  

أَنَّهُ طَلَّقَ امْرَأَتَهُ فِي الْحَيْضِ فَسَأَلَ عُمَرُ النَّبِيَّ ﷺ فَقَالَ مُرْهُ فَلْيُرَاجِعْهَا ثُمَّ لْيُطَلِّقْهَا طَاهِرًا أَوْ حَامِلاً