Sunnah.com reference : Book 8, Hadith 120English translation : Book 8, Hadith 1081Arabic reference : Book 8, Hadith 1070
Narrated Ibn 'Umar (RA):
In the time of Allah's Messenger (ﷺ), he divorced his wife while she was menstruating, so 'Umar asked Allah's Messenger (ﷺ) about that and he replied, "Command him to take her back and keep her till she is purified (from menses), has another period, and is then purified. If he then wishes he may keep her and if he wishes he may divorce her before having intercourse with her. That is the 'Iddah (period of waiting) which Allah commanded for the divorce of women."

[Agreed upon].
وَعَنِ اِبْنِ عُمَرَ ‏- رَضِيَ اَللَّهُ عَنْهُمَا { أَنَّهُ طَلَّقَ اِمْرَأَتَهُ ‏- وَهِيَ حَائِضٌ ‏- فِي عَهْدِ رَسُولِ اَللَّهِ ‏- صلى الله عليه وسلم ‏-فَسَأَلَ عُمَرُ رَسُولَ اَللَّهِ ‏- صلى الله عليه وسلم ‏-عَنْ ذَلِكَ ? فَقَالَ : " مُرْهُ فَلْيُرَاجِعْهَا , ثُمَّ لْيُمْسِكْهَا  حَتَّى تَطْهُرَ , ثُمَّ تَحِيضَ , ثُمَّ تَطْهُرَ , ثُمَّ إِنْ شَاءَ أَمْسَكَ بَعْدُ , وَإِنْ شَاءَ طَلَّقَ بَعْدَ أَنْ يَمَسَّ , فَتِلْكَ اَلْعِدَّةُ اَلَّتِي أَمَرَ اَللَّهُ أَنْ تُطَلَّقَ لَهَا اَلنِّسَاءُ }  مُتَّفَقٌ عَلَيْهِ  1‏ .‏‏1 ‏- صحيح .‏ رواه البخاري (5251)‏ ، ومسلم ( 1471)‏ (1)‏.‏