Zirr bin Hubaysh narrates from Sayyiduna Ali (may Allah be pleased with him) that when a free woman is married while (the man already has) a slave wife, he should allot two days of the turn to her and one day to the slave woman; and it is not appropriate for a slave woman to be married while there is a free wife.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14003