Musannaf Ibn Abi Shayba 10:735

Abu Bakr narrated to us, he said: Shababa narrated to us, from Malik ibn Anas, from al-Zuhri, from Sa'id ibn al-Musayyib and Sulayman ibn Yasar and others, that they used to say: The waiting period ('iddah) of the woman who seeks khul' (divorce initiated by the wife) is the waiting period of the divorced woman: three menstrual cycles (quru').