Musnad Ahmad 27:823

Rawh narrated to us, Hanzalah narrated to us, I heard Salim, and he was asked about a man who divorced his wife while she was menstruating? He said: "It is not permissible. Ibn Umar divorced his wife while she was menstruating, so the Messenger of Allah (peace and blessings of Allah be upon him) ordered him to take her back," so he took her back.