Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) said that a slave woman should not be married in addition to a free wife, while a free woman may be married in addition to a slave wife; and whoever is able to pay the dowry of a free woman should never marry a slave woman.
This chain is sahih (authentic).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14004