Musannaf Ibn Abi Shaybah

Hasan was asked: If a man divorces his wife and a slave marries her without his master's permission and also consummates the marriage, will she become lawful for the first husband? Hasan replied: He is not her husband. That is, she will not become lawful for the first husband.

The same is narrated from Hazrat Sha'bi. However, Hazrat Hakam says that this slave has become her husband. Now she can marry her former husband.

Hazrat Ata was asked that if a man gives his wife three divorces and a slave marries her without his master's permission, will she become lawful for the first husband? He replied that this marriage is not valid, therefore she cannot marry the first husband.

Ibrahim says that she cannot marry her first husband, because the marriage is not valid.

Hazrat Hammad says that she cannot marry her first husband until she enters into a valid marriage according to the Muslims.

Hazrat Tawus says that through an unlawful marriage, a woman does not become lawful for her first husband.

Hazrat Aamir says that a slave and a eunuch are, in any case, spouses.