Musnad Ahmad 27:984

Bahz narrated to us, Shu'bah narrated to us, Qatadah narrated to us, from Yunus ibn Jubayr, from Abdullah ibn Umar, that he divorced his wife while she was menstruating. Umar mentioned that to the Prophet, peace and blessings of Allah be upon him. The Messenger of Allah, peace and blessings of Allah be upon him, said: "Let him take her back until she is pure, then let him divorce her if he wishes."