Musannaf Ibn Abi Shaybah

Hazrat Ibn Abbas (may Allah be pleased with them both) says that a free man can marry only one slave woman.

Hazrat Harith says that a free man can marry four slave women. Hazrat Hammad says that he can marry only two slave women.

Hazrat Zuhri says that a free man can marry four slave women, and he can marry four Christian women. The same ruling applies to a slave.

Hazrat Qatadah says that Allah Almighty has made a slave-girl lawful for the person who fears sin and cannot afford the expenses of a free woman.