From Sahih Muslim (Hadith 3659):
Ibn 'Umar (Allah be pleased with them) reported that he divorced his wife while she was in the state of menses. 'Umar (Allah be pleased with him) made mention of it to Allah's Apostle (ﷺ) and he said:Command him to take her back, then divorce her when she is pure or she is pregnant
Reference: Muslim 3659