Musnad Ahmad 33:1033

Hassan narrated to us, Ibn Lahi'ah narrated to us, Abu al-Zubayr narrated to us, he said: I asked Jabir about a man who divorces his wife while she is menstruating. He said: Abdullah ibn Umar divorced his wife while she was menstruating. Umar came 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: "Let him take her back, for she is his wife."