Sunan Ibn Majah, Chapter: The Chapters on Divorce

"I asked Ibn Umar about a man who divorced his wife when she was menstruating. He said: 'Do you know Abdullah bin Umar? He divorced his wife when she was menstruating then Umar came to the Prophet (saw) (and told him what had happened). He ordered him to take her back.' I said: 'Will that be counted (as a divorce)?' He said: 'Do You think he was helpless and behaving foolishly? [i.e., Yes, it counts (as a divorce).]."'