Musnad Ahmad 27:985

Bahz narrated to us, Shu'bah narrated to us, he informed me—if Allah wills—Anas ibn Sirin, I heard Ibn Umar saying: Ibn Umar 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." He said: I said: "Does it count as a divorce?" He said: "What else?"