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 h

Ibn 'Umar (Allah be pleased with them) reported that he divorced his wife while she was in the state of menses. 'Umar (Allah be pleased with him) made mention of it to Allah's Apostle (ﷺ) and he said:

Command him to take her back, then divorce her when she is pure or she is pregnant.
وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَزُهَيْرُ بْنُ حَرْبٍ، وَابْنُ، نُمَيْرٍ - وَاللَّفْظُ لأَبِي بَكْرٍ - قَالُوا حَدَّثَنَا وَكِيعٌ، عَنْ سُفْيَانَ، عَنْ مُحَمَّدِ بْنِ عَبْدِ الرَّحْمَنِ، مَوْلَى آلِ طَلْحَةَ عَنْ سَالِمٍ، عَنِ ابْنِ عُمَرَ، أَنَّهُ طَلَّقَ امْرَأَتَهُ وَهْىَ حَائِضٌ فَذَكَرَ ذَلِكَ عُمَرُ لِلنَّبِيِّ صلى الله عليه وسلم فَقَالَ ‏ "‏ مُرْهُ فَلْيُرَاجِعْهَا ثُمَّ لْيُطَلِّقْهَا طَاهِرًا أَوْ حَامِلاً ‏"‏ ‏.‏
Reference : Sahih Muslim 1471h
In-book reference : Book 18, Hadith 8
USC-MSA web (English) reference : Book 9, Hadith 3479
  (deprecated numbering scheme)