Musnad Ahmad Hadith 6141

It is narrated from Ibn Umar (may Allah be pleased with them both) that I divorced my wife while she was in her menstrual period. Umar (may Allah be pleased with him) went and informed the Prophet (ﷺ) about this. The Prophet (ﷺ) became very angry and said: Tell him to take her back, then if he wishes to divorce her, he should do so during a period of purity before having intercourse with her. This is the method of divorce according to which Allah has permitted giving divorce. Since Ibn Umar (may Allah be pleased with them both) had already divorced her, it was counted, and Ibn Umar (may Allah be pleased with them both) took her back according to the command of the Prophet (ﷺ). He said: Tell me, do you want to prove him foolish and ignorant? (Why would the divorce not count?)