كتاب الطلاق
Read the English translation first, with the original Arabic directly underneath.
Abdullah b. 'Umar (Allah be pleased with them) reported:
I divorced my wife while she was in the state of menses. 'Umar (Allah be pleased with him) made mention of it to Allah's Messenger (ﷺ) and he was enraged and he said: Command him to take her back until she enters the second ensuing menses other than the one in which he divorced her and in case he deems proper to divorce her, he should pronounce divorce (finally) before touching her (in the period) when she is purified of her menses, and that is the prescribed period in regard to divorce as Allah has commanded. 'Abdullah made a pronouncement of one divorce and it was counted in case of divorce. 'Abdullah took her back as Allah's Messenger (ﷺ) had commanded him.
مُرْهُ فَلْيُرَاجِعْهَا حَتَّى تَحِيضَ حَيْضَةً أُخْرَى مُسْتَقْبَلَةً سِوَى حَيْضَتِهَا الَّتِي طَلَّقَهَا فِيهَا فَإِنْ بَدَا لَهُ أَنْ يُطَلِّقَهَا فَلْيُطَلِّقْهَا طَاهِرًا مِنْ حَيْضَتِهَا قَبْلَ أَنْ يَمَسَّهَا فَذَلِكَ الطَّلاَقُ لِلْعِدَّةِ كَمَا أَمَرَ اللَّهُ