(27) The Book of Divorce
كتاب الطلاق
(3) Chapter: What Should Be Done If The Husband Issues A Divorce When The Wife Is Menstruating
(3) باب مَا يَفْعَلُ إِذَا طَلَّقَ تَطْلِيقَةً وَهِيَ حَائِضٌ ‏‏
Sunan an-Nasa'i 3397 
It was narrated from Ibn 'Umar that he divorced his wife while she was menstruating. He mentioned that to the Prophet and he said:
"Tell him to take her back, then divorce her while she is pure (not menstruating) or pregnant."
أَخْبَرَنَا مَحْمُودُ بْنُ غَيْلاَنَ، قَالَ حَدَّثَنَا وَكِيعٌ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ مُحَمَّدِ بْنِ عَبْدِ الرَّحْمَنِ، مَوْلَى طَلْحَةَ عَنْ سَالِمِ بْنِ عَبْدِ اللَّهِ، عَنِ ابْنِ عُمَرَ، أَنَّهُ طَلَّقَ امْرَأَتَهُ وَهِيَ حَائِضٌ فَذَكَرَ ذَلِكَ لِلنَّبِيِّ صلى الله عليه وسلم فَقَالَ ‏ "‏ مُرْهُ فَلْيُرَاجِعْهَا ثُمَّ لْيُطَلِّقْهَا وَهِيَ طَاهِرٌ أَوْ حَامِلٌ ‏"‏ ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 3397
In-book reference : Book 27, Hadith 9
English translation : Vol. 4, Book 27, Hadith 3426