Musnad Ahmad 27:576

Muhammad ibn Ja‘far and ‘Abdullah ibn Bakr narrated to us, they said: Sa‘id narrated to us, from Qatadah, from Yunus ibn Jubayr, that he asked Ibn ‘Umar about a man who divorced his wife while she was menstruating. He said: "Do you know ‘Abdullah ibn ‘Umar? For he divorced his wife while she was menstruating. So ‘Umar went to the Messenger of Allah, peace and blessings of Allah be upon him, and informed him of that. The Messenger of Allah, peace and blessings of Allah be upon him, said: 'Command him to take her back, then if he wishes to divorce her, he may divorce her at the beginning of her waiting period.'" Ibn Bakr said: "Or at the beginning of her period of purity." I said to Ibn ‘Umar: "Is that divorce counted as a divorce?" He said: "Yes. What do you think if he was incapable and acted foolishly?"