Musnad Ahmad 2:223

Yazid narrated to us, 'Abd al-Malik informed us, from Anas ibn Sirin, who said: I said to Ibn 'Umar: "Tell me about your divorce of your wife." He said: "I divorced her while she was menstruating." He said: "So I mentioned that to 'Umar ibn al-Khattab, and he mentioned it to the Prophet, may Allah bless him and grant him peace. The Prophet, may Allah bless him and grant him peace, said: 'Command him to take her back, and when she is pure, let him divorce her during her period of purity.'" I said to him: "Did you count the one you divorced while she was menstruating?" He said: "And why would I not count it, even if I had been incapable and foolish?"