Narrated 'Abdullah bin 'Umar:

that he had divorced his wife while she was menstruating during the lifetime of Allah's Apostle . 'Umar bin Al-Khattab asked Allah's Apostle about that. Allah's Apostle ﷺ said, "Order him (your son) to take her back and keep her till she is clean and then to wait till she gets her next period and becomes clean again, whereupon, if he wishes to keep her, he can do so, and if he wishes to divorce her he can divorce her before having sexual intercourse with her;
and that is the prescribed period which Allah has fixed for the women meant to be divorced."

حَدَّثَنَا إِسْمَاعِيلُ بْنُ عَبْدِ اللَّهِ، قَالَ حَدَّثَنِي مَالِكٌ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ ـ رضى الله عنهما ـ أَنَّهُ طَلَّقَ امْرَأَتَهُ وَهْىَ حَائِضٌ عَلَى عَهْدِ رَسُولِ اللَّهِ ﷺ فَسَأَلَ عُمَرُ بْنُ الْخَطَّابِ رَسُولَ اللَّهِ ﷺ عَنْ ذَلِكَ فَقَالَ رَسُولُ اللَّهِ ﷺ " مُرْهُ فَلْيُرَاجِعْهَا، ثُمَّ لِيُمْسِكْهَا حَتَّى تَطْهُرَ ثُمَّ تَحِيضَ، ثُمَّ تَطْهُرَ، ثُمَّ إِنْ شَاءَ أَمْسَكَ بَعْدُ وَإِنْ شَاءَ طَلَّقَ قَبْلَ أَنْ يَمَسَّ، فَتِلْكَ الْعِدَّةُ الَّتِي أَمَرَ اللَّهُ أَنْ تُطَلَّقَ لَهَا النِّسَاءُ ".