Nafi' narrates from Ibn Umar (may Allah be pleased with them both) that he said: "It is not lawful for a man to have intercourse with a slave-girl except one whom, if he wishes, he may give away, if he wishes, sell, and if he wishes, set free, with no condition attached to her."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10831