Musnad Ahmad 27:819

'Abd al-Rahman ibn Mahdi and Bahz narrated to us, they said: Shu'bah narrated to us, from Anas ibn Sirin—Bahz said in his narration: Anas ibn Sirin informed me—that he heard Ibn 'Umar say: He divorced his wife while she was menstruating. So 'Umar asked the Prophet, may Allah bless him and grant him peace, and he said: "Command him to take her back, and when she is pure, let him divorce her." Bahz said: "Is it counted?"