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»