It is narrated from Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) that the waiting period of a slave woman, when she does not menstruate, is two months, and if she menstruates, her waiting period is two menstrual cycles. And it has been narrated to us through the chain of al-Hasan from Ali that Sayyiduna Ali (may Allah be pleased with him) said: "The waiting period of a slave woman is two menstrual cycles, and if she does not menstruate, then her waiting period is a month and a half."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15452