Nafi' narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both): When a slave gives his wife two divorces, she becomes unlawful for him until she marries another husband, whether she (the wife) is free or a slave woman; and the waiting period ('iddah) of a free woman is three menstrual cycles, while the waiting period of a slave woman is two menstrual cycles.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرجعة / 15164