It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Buy slaves, but beware of the Zanj (black Africans), for their lives are short and their provision is little."
Hadith Referenceالمعجم الأوسط / باب الألف / 1013
Hadith Takhrij«أخرجه الطبراني فى «الكبير» برقم: 10680»