Yahya related to me from Malik that he heard Ibn Shihab say, "Theidda of the divorced woman is reckoned by the menstrual cycles even ifshe is estranged ." (The reason the idda is normally reckoned by themenstrual cycle is to see whether the woman is pregnant or not.)
USC-MSA web (English) reference: Book 29, Hadith 61
Arabic reference: Book 29, Hadith 1222