Musnad Ahmad 27:341

Waki' narrated to us, Sufyan narrated to us, from Muhammad ibn Abd al-Rahman, the freed slave of the family of Talhah, from Salim—meaning Ibn Abdullah—from Ibn Umar, that he divorced his wife while she was menstruating. Umar asked the Prophet, peace and blessings of Allah be upon him, about that, and he said: "Command him to take her back, then let him divorce her while she is pure or pregnant."