Musnad Abu Ya'la al-Mawsili 109:1

Ishaq narrated to us, Abd al-Warith bin Sa'id narrated to us, from al-Qasim bin Abd al-Wahid, from Abdullah bin Muhammad bin Aqil, who heard Jabir say: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: "Indeed, when a slave marries without the permission of his master, he is a fornicator."