Yahya related to me from Malik that he had heard that Said ibnal-Musayyab and Ibn Shihab and Sulayman ibn Yasar all said, "The iddaof the woman with a khul divorce is three periods." (AR)
USC-MSA web (English) reference: Book 29, Hadith 60 (AR)
Arabic reference (AR): Book 29, Hadith 1221 (AR)