Musnad Abu Ya'la al-Mawsili 153:26

Abu Khaythama narrated to us, Waki’ narrated to us, from Sufyan, from Muhammad ibn ‘Abd al-Rahman, from Salim ibn ‘Abd Allah, from Ibn ‘Umar, that he divorced his wife while she was menstruating. So ‘Umar asked the Prophet, peace and blessings of Allah be upon him, and he said: "Command him to take her back, then let him divorce her while she is pure or pregnant."