It was narrated that Ibn 'Umar said:
"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.'"
English reference: Vol. 3, Book 10, Hadith 2019
Arabic reference: Book 10, Hadith 2097