Musnad Ahmad 30:2921

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Qatadah, from an-Nadr ibn Anas, from Bashir ibn Nahik, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, that he said: Regarding a slave owned by two men, where one of them emancipates his share, he said: "He must guarantee [the value of the other share]."