Musnad Ahmad 27:1681

Ya'qub narrated to us, the nephew of Ibn Shihab informed me, from his uncle, Salim ibn Abdullah informed us, that Abdullah ibn Umar said: I divorced my wife while she was menstruating. Umar mentioned that to the Messenger of Allah (peace and blessings of Allah be upon him). He said: The Messenger of Allah (peace and blessings of Allah be upon him) became angry, then said: "Let him take her back until she menstruates a future menstruation, other than the menstruation in which he divorced her. Then, if it appears to him to divorce her, let him divorce her while she is pure from her menstruation, before he touches her. That is the divorce for the waiting period as Allah the Exalted commanded." Abdullah had divorced her with one pronouncement, so it was counted as one of her divorces, and Abdullah took her back as he was commanded.