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." (ID)
USC-MSA web (English) reference: Book 29, Hadith 60 (ID)
Arabic reference (ID): Book 29, Hadith 1221 (ID)