Sunan Ibn Majah Book 19, Hadith 18
“Whoever frees a slave who has some wealth, the slave's wealth belongs to him, unless the master stipulates that it will belong to him.”
“Whoever frees his share of a slave, the price of the slave should be fairly evaluated, and he (the partner who initiated this process) should free him (in full, by giving the rest of his price to the other co-owners), if he has enough wealth to do so. Otherwise, he will have freed whatever he freed.”
“O Umair, I have set you free in a good way. I heard the Messenger of Allah (ﷺ) say: 'Any man who frees a slave and does not say anything about his (the slave's) wealth, it belongs to him (the slave).' So tell me, how much wealth do you have?”
Another chain reports a similar hadith.