Maliks Muwatta Book 29, Hadith Number 61.
Section : Idda of Divorce and Divorce of Menstruating Women.
Yahya related to me from Malik that he heard Ibn Shihab say, “The idda of the divorced woman is reckoned by the menstrual cycles even if she is estranged.” (The reason the idda is normally reckoned by the menstrual cycle is to see whether the woman is pregnant or not.)