Musnad Ahmad 27:53

Isma'il narrated to us, Ayyub narrated to us, from Nafi', that Ibn Umar divorced his wife with one pronouncement while she was menstruating. So Umar asked the Prophet, peace and blessings of Allah be upon him, about it. He ordered him to take her back, then wait until she menstruates another menstruation, then wait until she is pure, then divorce her before he touches her. He said: "And that is the waiting period which Allah, the Mighty and Majestic, commanded that women be divorced for." Whenever Ibn Umar was asked about a man who divorces his wife while she is menstruating, he would say: As for me, I divorced her with one or two [pronouncements], then "the Messenger of Allah, peace and blessings of Allah be upon him, ordered him to take her back, then wait until she menstruates another menstruation, then wait until she is pure, then divorce her before he touches her." As for you, if you divorced her with three [pronouncements], you have disobeyed Allah regarding what He commanded you of divorcing your wife, and she has become irrevocably separated from you.