Musnad Ahmad 27:872

'Abd al-Wahhab ibn 'Abd al-Majid narrated to us, from Ayyub, from Nafi', that Ibn 'Umar divorced his wife while she was menstruating. So 'Umar asked the Prophet, peace and blessings of Allah be upon him, and he ordered him to take her back, then wait for her until she menstruates another menstruation, then wait for her until she is pure, then divorce her before he touches her. That is the waiting period which Allah has commanded that women be divorced for. When Ibn 'Umar was asked about a man who divorces his wife while she is menstruating, he would say: "Either you divorced her once or twice, for the Messenger of Allah, peace and blessings of Allah be upon him, ordered him to take her back, then wait for her until she menstruates another menstruation, then wait for her until she is pure, then divorce her if he does not wish to keep her. Or you divorced her three times, then you have disobeyed Allah the Exalted in what He commanded you regarding the divorce of your wife, and she has become separated from you, and you have become separated from her."