Muwatta Imam Malik Book of Setting Free and Wala', Hadith 2

Malik said, "A master who frees a slave of his and settles his emancipation so that his testimony is permitted, his inviolability complete, and his right to inherit confirmed, cannot impose stipulations on him like what he imposes on a slave about property or service, nor get him to do anything of slavery, because the Messenger of Allah ﷺ 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 give his partners their shares so the slave is completely free."
Malik commented, "If he owns the slave completely, it is more proper to free him completely and not mingle any slavery with it."