His father divorced his wife during her menses, so Umar asked the Prophet about that and he said: "Tell him to take her back, then let him divorce her while she is pure or pregnant."
Jami` at-Tirmidhi Book 13, Hadith 2
← Previous Hadith "I asked Ibn Umar about a man who divorced his wife while she was menstruating. So he said: 'Don't you know Abdullah bin Umar?' Indeed he divorced his wife while she was menstruating, so Umar asked the Prophet about that, and he ordered him to take her back.'" He said: "I said: And that divorce is counted? He said: And that divorce is counted? He said: 'What else would you think if he was helpless and foolish?'"

Jami` at-Tirmidhi Book 13, Hadith 1 Next Hadith → 'O Messenger of Allah! I irrevocably divorced my wife.' So he said: 'What did you intend by that?' I said: 'One (divorce).' He said: '(Do you swear) By Allah?' I said: 'By Allah.' He said: 'Then it is as you intended.'"

Jami` at-Tirmidhi Book 13, Hadith 3