Musnad Ahmad 27:1659

Muhammad ibn Ubayd narrated to us, Abd al-Malik—meaning Ibn Abi Sulayman—narrated to us, from Anas ibn Sirin, from Ibn Umar, who said: I asked him about his wife whom he divorced during the era of the Messenger of Allah (peace and blessings of Allah be upon him). He said: I divorced her while she was menstruating. I mentioned that to Umar, and Umar mentioned it to the Prophet (peace and blessings of Allah be upon him). The Prophet (peace and blessings of Allah be upon him) said: "Command him to take her back, then when she is pure, he may divorce her in her state of purity, according to the Sunnah." He said: So I did that. Anas said: I asked him: Did you count the one you divorced while she was menstruating? He said: "And why would I not count it? If I was incapable and foolish."