Musnad Ahmad 30:2375

Isma'il narrated to us, Sa'id ibn Abi 'Arubah 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, who said: "Whoever emancipates his share of a slave, his freedom is [to be paid] from his wealth, if he has wealth. If he does not have wealth, the slave is to work to earn his freedom, without being overburdened."