he divorced his wife when she was menstruating, and 'Umar mentioned that to the Prophet (ﷺ). He said: "Tell him to take her back then divorce her when she is pure (not menstruating) or pregnant."
Sunan Ibn Majah Book 10, Hadith 8
← Previous Hadith "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 (ﷺ) (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? ."'

Sunan Ibn Majah Book 10, Hadith 7 Next Hadith → "I said to Fatimah bint Qais: 'Tell me about your divorce.' She said: 'My husband divorced me three times when he was leaving for Yemen, and the Messenger of Allah (ﷺ) allowed that."'

Sunan Ibn Majah Book 10, Hadith 9