Al-Hasan (may Allah have mercy on him) said that the Messenger of Allah (peace and blessings be upon him) forbade that a slave woman be married in addition to a free wife.
This is mursal (a report missing the Companion in its chain), except that it is in line with the meaning of the Book (of Allah), and with it is the view of a group of the Companions (may Allah be pleased with them).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14002