Abu Zakariyya al Azdi: Tarikh al Mawsil (History of Mosul) (d945)
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Taken from: Manufacturing and labour by Michael G. Morony
…… there were 4,000 Zanj slaves headed by a qa'id (a commander) in the army of the provincial governor of Mawsil (Mosul)
during the Caliphate of Abu Ja'far al-Mansur (754-775 A.D.). (1)
(1) this will be repeated by Al Maqrizi (1441)