Musnad Ahmad 30:2977

Yahya narrated to us, Ibn Abi Arubah narrated to us, from Qatadah, from An-Nadr ibn Anas, from Bashir ibn Nahik, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever emancipates his share of a slave, he is responsible for the full emancipation of that slave from his own wealth. If he does not have wealth, the slave is to work to earn his freedom, without being overburdened."