Nafi' narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) concerning a slave woman who is married to a free man: two divorces will separate her, and she will observe a waiting period of two menstrual cycles; and when a free woman is married to a slave, she will be separated by two divorces and will observe a waiting period of three menstrual cycles.
(b) Salim narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the divorces of the two of them are reduced because of slavery. [Sahih]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرجعة / 15165