Musnad Ahmad 27:1055

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Qatadah, who said: I heard Yunus ibn Jubayr, who said: I heard Ibn Umar saying: I divorced my wife while she was menstruating. Umar came to the Prophet, may Allah bless him and grant him peace, and mentioned that to him. He said: "Let him take her back, and when she becomes pure, then if he wishes, let him divorce her." He said: So I said to Ibn Umar: "Does it count as a divorce?" He said: "What would prevent it? Yes. What do you think if he was incapable and acted foolishly?"