كتاب الأحكام
Read the English translation first, with the original Arabic directly underneath.
Narrated `Abdullah bin `Umar:
That he had divorced his wife during her menses. `Umar mentioned that to the Prophet. Allah's Messenger (ﷺ) became angry and said, "He must take her back (his wife) and keep her with him till she becomes clean from her menses and then to wait till she gets her next period and becomes clean again from it and only then, if he wants to divorce her, he may do so."
لِيُرَاجِعْهَا، ثُمَّ لْيُمْسِكْهَا حَتَّى تَطْهُرَ، ثُمَّ تَحِيضَ فَتَطْهُرَ، فَإِنْ بَدَا لَهُ أَنْ يُطَلِّقَهَا فَلْيُطَلِّقْهَا