Sahih Muslim Book 9, Hadith Number 3581.

Chapter: Emancipating a slave.

Abu Huraira (Allah be pleased with him) reported Allah’s Prophet (may peace be upon him) as saying: If anyone emancipates a share in a slave, he is to be completely emancipated if he has money; but if he has none, the slave will be required to work to pay for his freedom, but must not be over-burdened.