Sahih Muslim
The Book of Oaths (Kitab Al-Aiman)
Book Number: 015 / Hadith Number: 4103
Ibn 'Umar reported Allah's Messenger (may peace be upon him) as saying: He who emancipates his share in the slave, it is his responsibility to secure full freedom for him provided he (the slave) has enough money to pay the (remaining) price, but it he has not so much money he would be emancipated to the extent that the first man emancipated.