Yahya related to me from Malik from Nafi that Abdullah ibn Umarsaid, "When the slave divorces his wife twice, she is haram for himuntil 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 aslave-girl is two periods.
USC-MSA web (English) reference: Book 29, Hadith 50
Arabic reference: Book 29, Hadith 1210