Musnad Ahmad 27:821

Rawh narrated to us, Muhammad ibn Abi Hafsah narrated to us, Ibn Shihab narrated to us, from Salim, from his father, that he divorced his wife while she was menstruating. He mentioned that to Umar, so Umar went to the Messenger of Allah, peace and blessings of Allah be upon him, and informed him. The Messenger of Allah, peace and blessings of Allah be upon him, said: "Let him keep her until she menstruates a menstruation other than this one, then she becomes pure. Then, if it appears to him to divorce her, let him divorce her as Allah, the Mighty and Majestic, commanded him. And if it appears to him to keep her, let him keep her."