Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says: "When a free woman is the wife of a slave, the woman can be given two divorces, and her waiting period (‘iddah) will be three menstrual cycles. And when a slave woman is the wife of a free man, she can be given two divorces and her waiting period, according to her status as a slave woman (i.e., two menstrual cycles), will be observed."