"O Prophet! When you divorce women, divorce them at their 'Iddah (prescribed periods)." Ibn 'Abbas, may Allah be pleased with him, said: "Before their 'Iddah elapses."
Sunan an-Nasa'i Book 27, Hadith 5
← Previous Hadith "What did you think about a man who divorces his wife when she is menstruating?" He said to him: "Abdullah bin 'Umar divorced his wife when she was menstruating during the time of the Messenger of Allah. 'Umar asked the Messenger of Allah (about that) and said: 'Abdullah bin 'Umar has divorced his wife while she was menstruating.' The Messenger of Allah said: 'Let him take her back.' So he made me take her back. He said: 'When she becomes pure, let him divorce her or keep her.' Ibn 'Umar said: 'The Prophet said: 'O Prophet! When you divorce women, divorce them before their 'Iddah (prescribed period) elapses.'"

Sunan an-Nasa'i Book 27, Hadith 4 Next Hadith → "The Sunnah divorce is a divorce issued when she is pure (not menstruating) without having had intercourse with her. If she menstruates and becomes pure again, give her another divorce, and if she menstruates and becomes pure again, give her another divorce, then after that, she should wait for another menstrual cycle." (One of the narrators) Al-A'mash said: "I asked Ibrahim, and he said something similar."

Sunan an-Nasa'i Book 27, Hadith 6