Musannaf Ibn Abi Shayba 9:960

Yazid ibn Harun narrated to us, he said: Hammam informed us, from al-Qasim ibn Abd al-Wahid, from Abd Allah ibn Muhammad ibn Aqil, from Jabir, he said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Any slave who marries or enters into a marriage contract without the permission of his masters is a fornicator."