Musnad Ahmad 33:914

Yahya ibn Sa'id al-Umawi narrated to us, from Ibn Jurayj, [who said]: 'Abdullah ibn Muhammad ibn 'Aqil informed me, from Jabir ibn 'Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Any slave who marries without the permission of his master, he is a fornicator."