Musnad Ahmad 33:975

Yazid ibn Harun narrated to us, Hammam ibn Yahya informed us, from al-Qasim ibn Abd al-Wahid, from Abdullah ibn Muhammad ibn Aqil, from Jabir ibn Abdullah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Any slave who marries without the permission—or he said: who weds without the permission—of his masters, he is an adulterer."