Musnad Ahmad 27:779

Waki’ narrated to us, Sufyan narrated to us, from Muhammad ibn ‘Abd al-Rahman, the freed slave of the family of Talhah, from Salim ibn ‘Abd Allah, from 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, and he said: "Command him to take her back, then let him divorce her when she is pure or pregnant."