Malik related to me from Nafi from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace,said, "If a man frees his share of a slave and has enough money to cover the full price of the slave justly evaluated for him, he must buy out his partners so that the slave is completely freed. If he doesn't have the money, he partially frees him.
USC-MSA web (English) reference: Book 38, Hadith 1
Arabic reference: Book 38, Hadith 1467