Musnad Ahmad 27:1040

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Anas ibn Sirin, that he heard Ibn ‘Umar say: "I divorced my wife while she was menstruating. So ‘Umar came to the Prophet, may Allah bless him and grant him peace, and informed him. He said: 'Command him to take her back, then when she is pure, let him divorce her [if he wishes].' I said to Ibn ‘Umar: 'Do you count that as a divorce?' He said: 'What else?'"