Maliks Muwatta Book 29, Hadith Number 50.

Section : The Divorce of the Slave.

Yahya related to me from Malik from Nafi that Abdullah ibn Umar said, “When the slave divorces his wife twice, she is haram for him until she has married another husband, whether she is free or a slave. The idda of a free woman is three menstrual periods, and the idda of a slave-girl is two periods.