It was narrated from 'Aishah that:
a man bought a slave and put him to work, then he found some defect in him, so he returned him. He (the seller) said: "O Messenger of Allah he put my slave to work." The Messenger of Allah (ﷺ) said: "A slave's earnings belong to his guarantor."
English reference: Vol. 3, Book 12, Hadith 2243
Arabic reference: Book 12, Hadith 2328