Musnad Ahmad 27:850

'Abd al-Rahman narrated to us, from Malik, from Nafi', 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 keep her until she becomes pure, then she menstruates, then she becomes pure. Then, if he wishes, he may divorce her, and if he wishes, he may keep her. For that is the waiting period which Allah has commanded that women be divorced for."