Atiyyah al-Awfi (may Allah have mercy on him) said: I heard Sayyiduna Ibn Abbas (may Allah be pleased with them both) say: "The Messenger of Allah (peace and blessings be upon him) forbade the earnings of slave-girls (from immorality)." I asked Ibn Abbas (may Allah be pleased with them both): Why did he (peace and blessings be upon him) forbid it? He said: "For fear that they would become unable (to earn), and then be forced (into immorality)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12619