Sahih Muslim Book 27, Hadith 71
It is good for a slave that he worships Allah well, and serves his master (well). It is good for him.
The above hadith has been reported through another chain of transmitters on the authority of Abu Huraira.
He who gives up his share in a slave, and has enough money to pay the full price of the slave, then full emancipation devolves upon him; but if he has not the money, then he emancipated what he emancipated.