"Divorce according to the Sunnah means divorcing her when she is pure, ( i.e., not menstruating) and without having had intercourse with her (during that cycle)."
Sunan Ibn Majah Book 10, Hadith 5
← Previous Hadith "I divorced my wife when she was menstruating. 'Umar mentioned that to the Messenger of Allah and he said: 'Tell him to take her back until she becomes pure (i.e., her period ends), then she has her period (again), then she becomes pure (again), then if he wishes he may divorce her before having sexual relations with her, and if he wishes he may keep her. This is the waiting period that Allah has enjoined.'"

Sunan Ibn Majah Book 10, Hadith 4 Next Hadith → "Divorce according to the Sunnah means divorcing her with one divorce in each cycle when she is pure, then when she becomes pure the third time, then he pronounces divorce again, and after that she must wait one more menstrual cycle."

Sunan Ibn Majah Book 10, Hadith 6