It was narrated from Ibn 'Umar that he divorced his wife when she was menstruating, and the Messenger of Allah told him to take her back.
Sunan an-Nasa'i Book 27, Hadith 172
← Previous Hadith "If it is the first or second divorce, the Messenger of Allah would tell him to take her back and keep her until she has menstruated again and purified herself, then divorce her before having intercourse with her. But if it was three simultaneous divorces, then you have disobeyed Allah with regard to the way in which divorce should be conducted and your wife has become irrevocably divorced."

Sunan an-Nasa'i Book 27, Hadith 171 Next Hadith → "Do you know 'Abdullah bin 'Umar?" He said: "Yes." He said: "He divorced his wife when she was menstruating, and 'Umar went to the Prophet and told him about that. He ordered him to take her back until she became pure," and I did not hear him adding anything to that.

Sunan an-Nasa'i Book 27, Hadith 173