He divorced his wife while she was menstruating, so Umar asked the Prophet ﷺ about it. He replied, "Order him to take her back, then keep her until she becomes pure, then menstruates again, then becomes pure once more. After that, if he wishes, he may divorce her, and if he wishes, he may keep her. That is the waiting period during which Allah commanded that women be divorced."
حدثنا عبد الرحمن عن مالك عن نافع
أنه طلق امرأته وهي حائض فسأل عمر النبي ﷺ فقال مره فليراجعها ثم يمسكها حتى تطهر ثم تحيض ثم تطهر ثم إن شاء طلقها وإن شاء أمسكها فتلك العدة التي أمر الله أن يطلق لها النساء
Its isnad is sahih. It is in Muwatta at greater length than this, 2:96, and the reference to the narration in Muwatta was already noted in the commentary on hadith 4500. The hadith has previously appeared at length and in abbreviated form several times, the most recent being 5272.