Musnad Ahmad 27:672

Isma'il narrated to us, from Yunus, from Muhammad ibn Sirin, from Yunus ibn Jubayr, who said: I asked Ibn Umar about a man who divorces his wife while she is menstruating. He said: "Do you know Abdullah ibn Umar?" I said: "Yes." He said: "He divorced his wife while she was menstruating, so Umar came to the Prophet, may Allah bless him and grant him peace, and asked him. The Prophet, may Allah bless him and grant him peace, commanded him to take her back, then divorce her [when she is pure], so that she may begin her waiting period."