كتاب الطلاق
18
The Book of Divorce
(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
(1)
باب تَحْرِيمِ طَلاَقِ الْحَائِضِ بِغَيْرِ رِضَاهَا وَأَنَّهُ لَوْ خَالَفَ وَقَعَ الطَّلاَقُ وَيُؤْمَرُ بِجْعَتِهَا
Sahih Muslim 1471 n

Ibn 'Umar (Allah be pleased with them) reported:

I divorced my wife while she was in the state of menses. 'Umar (Allah he pleased wish him) came toAllah's Apostle (ﷺ) and made mention of that to him, whereupon Allah's Apostle (ﷺ) told that be should take her back, and when she is pure he may divorce her. if he would so wish. I (one of the narrators) said to Ibn 'Umar (Allah be pleased with them): Did you count (this pronouncement of divorce) in her case? He said: What (after all) prevents him from doing so? Do you find him (Ibn Umar) either helpless or foolish?
حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، وَابْنُ، بَشَّارٍ قَالَ ابْنُ الْمُثَنَّى حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ قَتَادَةَ، قَالَ سَمِعْتُ يُونُسَ بْنَ جُبَيْرٍ، قَالَ سَمِعْتُ ابْنَ عُمَرَ، يَقُولُ طَلَّقْتُ امْرَأَتِي وَهْىَ حَائِضٌ فَأَتَى عُمَرُ النَّبِيَّ صلى الله عليه وسلم فَذَكَرَ ذَلِكَ لَهُ فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏ "‏ لِيُرَاجِعْهَا ‏.‏ فَإِذَا طَهَرَتْ فَإِنْ شَاءَ فَلْيُطَلِّقْهَا ‏"‏ ‏.‏ قَالَ فَقُلْتُ لاِبْنِ عُمَرَ أَفَاحْتَسَبْتَ بِهَا قَالَ مَا يَمْنَعُهُ ‏.‏ أَرَأَيْتَ إِنْ عَجَزَ وَاسْتَحْمَقَ.
Reference : Sahih Muslim 1471 n
In-book reference : Book 18, Hadith 14
USC-MSA web (English) reference : Book 9, Hadith 3485
  (deprecated numbering scheme)