Musannaf Ibn Abi Shayba 13:1219

Abu Bakr narrated to us, he said: Mu'adh ibn Mu'adh narrated to us, from Ash'ath, from al-Hasan, regarding a man who bought a slave and discovered a defect, while he had already utilized the slave's labor (or gained profit from him). He said: The profit (or yield) belongs to the purchaser.