It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Buy slaves and share with them in their earnings, that is, in what they earn, and beware of the Zanj, for their lives are short and their provision is scant."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10634