From Sahih Muslim (Hadith 3660):
Ibn 'Umar (Allah be pleased with them) reported that he divorced his wife while she was in her menses. 'Umar (Allah be pleased with him) asked Allah's Apostle (ﷺ) about that, and he said:Command him to take her back until she is pure and then she enters the second menses and then becomes pure. Then either divorce her (finally) or retain her
Reference: Muslim 3660