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Ibn ‘Umar said that he divorced his wife while she was menstruating. ‘Umar mentioned the matter to the Prophet(ﷺ). He (the Prophet) said “Order him, he must take her back and divorce her when she is purified (from menstrual discharge) or she is pregnant.”
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عُثْمَانُ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا وَكِيعٌ، عَنْ سُفْيَانَ، عَنْ مُحَمَّدِ بْنِ عَبْدِ الرَّحْمَنِ، مَوْلَى آلِ طَلْحَةَ عَنْ سَالِمٍ، عَنِ ابْنِ عُمَرَ، أَنَّهُ طَلَّقَ امْرَأَتَهُ وَهِيَ حَائِضٌ فَذَكَرَ ذَلِكَ عُمَرُ لِلنَّبِيِّ صلى الله عليه وسلم فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " مُرْهُ فَلْيُرَاجِعْهَا ثُمَّ لْيُطَلِّقْهَا إِذَا طَهُرَتْ أَوْ وَهِيَ حَامِلٌ " .
sunnah.com