It is narrated from Sayyidah Aishah (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) said: "A slave woman is to be divorced with two divorces, and she is to observe a waiting period of two menstrual cycles."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15456