It was narrated that Ibn 'Umar said: "I divorced my wife when she was menstruating. 'Umar mentioned that to the Messenger of Allah (ﷺ) and he said: 'Tell him to take her back until she becomes pure (i.e., her period ends), then she has her period (again), then she becomes pure (again), then if he wishes he may divorce her before having sexual relations with her, and if he wishes he may keep her. This is the waiting period that Allah has enjoined.'"
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ إِدْرِيسَ، عَنْ عُبَيْدِ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ طَلَّقْتُ امْرَأَتِي وَهِيَ حَائِضٌ فَذَكَرَ ذَلِكَ عُمَرُ لِرَسُولِ اللَّهِ ـ صلى الله عليه وسلم ـ فَقَالَ " مُرْهُ فَلْيُرَاجِعْهَا حَتَّى تَطْهُرَ ثُمَّ تَحِيضَ ثُمَّ تَطْهُرَ ثُمَّ إِنْ شَاءَ طَلَّقَهَا قَبْلَ أَنْ يُجَامِعَهَا وَإِنْ شَاءَ أَمْسَكَهَا فَإِنَّهَا الْعِدَّةُ الَّتِي أَمَرَ اللَّهُ " .
sunnah.com