Musnad Ahmad 27:715

Yahya narrated to us, from Ubaydullah, Nafi’ informed me, from Ibn Umar, that he divorced his wife while she was menstruating. Umar came to the Prophet, peace and blessings of Allah be upon him, and asked him for a legal ruling. He said: "Command Abdullah to take her back until she is pure from this menstruation of hers, then she menstruates another menstruation. Then, when she is pure, let him divorce her before he has intercourse with her, or let him keep her. For that is the waiting period which Allah commanded that women be divorced for."