Musannaf Ibn Abi Shaybah

Hazrat Saeed bin Musayyib states that Hazrat Umar (may Allah be pleased with him) forbade Arabs from marrying slave women. He gave a ruling of six camels regarding Arabs who married slave women; men and women are equal in this matter, and the same ruling applies to the clients (mawali) when it is not known. Hazrat Zuhri states that Arabs and mawali are not equal in lineage.