It is narrated from Abdullah bin Utbah that Umar bin Khattab, may Allah be pleased with him, said: "A slave may marry two women, and he will give only two divorces, and the waiting period (‘iddah) of a slave woman will be two menstrual cycles; if she does not menstruate, then the waiting period will be two or one and a half months."
Hadith Referenceمسند الإمام الشافعي / كتاب العدد والسكنى والنفقات / 1294
Hadith Takhrijصحيح: اخرجه البيهقي: 4257 - وفى المعرفة السنن والآثار له (4639) وعبد الرزاق (12872)، 13134 - والدارقطني: (3/ 308)۔