He divorced his wife while she was menstruating. Umar mentioned this to the Messenger of Allah ﷺ, and the Messenger of Allah ﷺ became angry about it, then said, "Let him take her back and keep her until she becomes pure, then menstruates again and becomes pure once more. If he then decides to divorce her, let him do so while she is in a state of purity and before he has touched her. That is the waiting period as Allah has commanded."
أنه طلق امرأته وهي حائض، فذكرعمر الرسول الله ﷺ، فتغيظ فيه رسول الله ﷺ، ثم قال: ليراجعها، ثم يمسكها حتى تطهر، ثم تحيض فتطهر، فإن بدا له أن يطلقها فليطلقهاطاهرا قبل أن يمسها، فتلك العدة كما أمر الله.
Bukhari narrated it in the tafsir (4908), and Muslim narrated it in the divorce chapter (1471:4), both of them by way of Ibn Shihab Zuhri, who said, Salim bin Abdullah told me, that Abdullah bin Umar said, and then he related it. The wording given is Bukhari's, and Muslim's wording is similar.