It was narrated from Ishaq bin Ibrahim, from his grandfather 'Umair, who was the freed slave of Ibn Mas'ud, that `Abdullah said to him:
"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?"
English reference: Vol. 3, Book 19, Hadith 2530
Arabic reference: Book 19, Hadith 2626