Jabir bin Abdullah (may Allah be pleased with them both) says that he heard the Prophet (peace and blessings be upon him) say: "Any slave who marries without the permission of his master is a fornicator."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13729
Ibn Umar (may Allah be pleased with them both) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "When a slave marries without the permission of his master, his marriage is void."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13731
Sayyiduna Ibn Umar (may Allah be pleased with them both) considered the marriage of a slave without his master's permission to be fornication, and he would say: Whoever arranges such a marriage is to be punished.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13732
Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) used to say: When a slave marries without the permission of his master, the divorce is in the slave's hands. It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) concerning a slave who married a free woman without his master's permission: She is a woman who has made her private part lawful.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13733