Sayyidah Aishah (may Allah be pleased with her) says that the Messenger of Allah (peace and blessings be upon him) said: "The divorces of a slave are two, and that woman is not lawful for him until she marries another husband; and the waiting period of a slave woman is two menstrual cycles. A free woman may be married in the presence of a slave woman (i.e. a wife), but a slave woman may not be married in the presence of a free woman (i.e. a wife)."
This is how he worded it: "The divorces of a slave are two."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرجعة / 15169