Maliks Muwatta Book 29, Hadith Number 79.
Section : General Section on Divorce.
Yahya related to me from Malik that Abdullah ibn Dinar said, “I heard Abdullah ibn Umar recite from the Qur’an, ‘Prophet! When you divorce women, divorce them at the beginning of their idda.'”
Malik said, “He meant by that, to make one pronouncement of divorce at the beginning of each period of purity.”