The Book of Divorce
18/1. Chapter:
The prohibition of divorcing a menstruating woman without her consent; If a man breaks this rule, it still counts as a divorce, and he should be ordered to take her back
Sahih Muslim
حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، وَابْنُ، بَشَّارٍ قَالَ ابْنُ الْمُثَنَّى حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ أَنَسِ بْنِ سِيرِينَ، أَنَّهُ سَمِعَ ابْنَ عُمَرَ، قَالَ طَلَّقْتُ امْرَأَتِي وَهْىَ حَائِضٌ فَأَتَى عُمَرُ النَّبِيَّ صلى الله عليه وسلم فَأَخْبَرَهُ فَقَالَ " مُرْهُ فَلْيُرَاجِعْهَا ثُمَّ إِذَا طَهَرَتْ فَلْيُطَلِّقْهَا " . قُلْتُ لاِبْنِ عُمَرَ أَفَاحْتَسَبْتَ بِتِلْكَ التَّطْلِيقَةِ قَالَ فَمَهْ .
Anas b. Sirin reported that he had heard Ibn 'Umar (Allah be pleased with them) as saying. I divorced my wife while she was in the state of menses. 'Umar (Allah be pleased with him) came to Allah's Apostle (ﷺ) and informed him about it, whereupon he (Allah's Apostle) said: Command him to take her back and when she is pure, then divorce her. I said to Ibn 'Umar Allah be pleased with them): Did you count that pronouncement of divorce? He said: Why not?
Reference : Sahih Muslim 1471pIn-book reference : Book 18, Hadith 16USC-MSA web (English) reference : Book 9, Hadith 3487