"I asked Ibn 'Umar about a man who divorced his wife while she was menstruating. He said: 'Do you know 'Abdullah bin 'Umar?' He divorced his wife while she was menstruating, and 'Umar asked the Prophet about that, and he told him to take her back, then wait for the right time. I said to him: 'Was that divorce counted?' He said: 'Be quiet! What do you think if some becomes helpless and behaves foolishly?'" (Using translation from Nasāʾī 3399)  

هَلْ تَعْرِفُ عَبْدَ اللهِ بْنَ عُمَرَ فَإِنَّهُ طَلَّقَ امْرَأَتَهُ وَهِيَ حَائِضٌ فَسَأَلَ عُمَرُ النَّبِيَّ ﷺ فَأَمَرَهُ أَنْ يُرَاجِعَهَا فَقُلْتْ لَهُ فَتَعْتَدُّ بِتِلْكَ التَّطْلِيقَةِ؟ فَقَالَ «مَهْ أَرَأَيْتِ إِنْ عَجَزَ وَاسْتَحْمَقَ»  

I said to Ibn'Umar (Allah be pleased with them): A person divorcedhis wife while she was in the state of menses, whereupon he said: Do you know 'Abdullah b. Umar (Allah be pleased with them), for he divorced his wife in the state of menses. 'Umar (Allah be pleased with him) came to Allah's Apostle ﷺ and asked him, and he (the Holy Prophet) commanded him that he should take her back, and she started her 'Idda. I said to him: When a person divorces his wife, and she is in the state of menses, should that pronouncement of divorce be counted? He said: Why not, was he hopless or foolish? (Using translation from Muslim 1471m)   

أَتَعْرِفُ عَبْدَ اللهِ بْنَ عُمَرَ فَإِنَّهُ طَلَّقَ امْرَأَتَهُ وَهِيَ حَائِضٌ فَأَتَى عُمَرُ النَّبِيَّ ﷺ فَسَأَلَهُ فَأَمَرَهُ أَنْ يُرَاجِعَهَا ثُمَّ تَسْتَقْبِلُ عِدَّتَهَا قُلْتُ لَهُ إِذَا طَلَّقَ الرَّجُلُ امْرَأَتَهُ وَهِيَ حَائِضٌ أَتَعْتَدُّ بِتِلْكَ التَّطْلِيقَةِ؟ فَقَالَ «مَهْ أَوَإِنْ عَجَزَ وَاسْتَحْمَقَ»