‘Abd Allah (bin Umar) said that he divorced his wife while she was menstruating. ‘Umar mentioned the matter to the Messenger of Allah(ﷺ). The Messenger of Allah(ﷺ) became angry and said “Command him, he must take her back and keep her back till she is purified, then has another menstrual period and is purified. Then if he desires he may divorce her during the period of purity before he has intercourse with her. This is the divorce for waiting period as commanded by Allaah, the Exalted.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا أَحْمَدُ بْنُ صَالِحٍ، حَدَّثَنَا عَنْبَسَةُ، حَدَّثَنَا يُونُسُ، عَنِ ابْنِ شِهَابٍ، أَخْبَرَنِي سَالِمُ بْنُ عَبْدِ اللَّهِ، عَنْ أَبِيهِ، أَنَّهُ طَلَّقَ امْرَأَتَهُ وَهِيَ حَائِضٌ فَذَكَرَ ذَلِكَ عُمَرُ لِرَسُولِ صلى الله عليه وسلم فَتَغَيَّظَ رَسُولُ اللَّهِ صلى الله عليه وسلم ثُمَّ قَالَ " مُرْهُ فَلْيُرَاجِعْهَا ثُمَّ لْيُمْسِكْهَا حَتَّى تَطْهُرَ ثُمَّ تَحِيضَ فَتَطْهُرَ ثُمَّ إِنْ شَاءَ طَلَّقَهَا طَاهِرًا قَبْلَ أَنْ يَمَسَّ فَذَلِكَ الطَّلاَقُ لِلْعِدَّةِ كَمَا أَمَرَ اللَّهُ عَزَّ وَجَلَّ " .
sunnah.com