Maliks Muwatta Book 29, Hadith Number 53.

Section : Idda of Divorce and Divorce of Menstruating Women.

Yahya related to me from Malik from Nafi that Abdullah ibn Umar divorced his wife while she was menstruating in the time of the Messenger of Allah, may Allah bless him and grant him peace, Umar ibn al-Khattab asked the Messenger of Allah, may Allah bless him and grant him peace, about it. The Messenger of Allah, may Allah bless him and grant him peace, said, “Go and tell him to take her back and keep her until she is purified and then has a period and then is purified. Then if he wishes, he an keep her, and if he wishes he should divorce her before he has intercourse with her. That is the idda which Allah has commanded for women who are divorced.”