From Sahih Muslim (Hadith 4309):
Abu Mas'ud reported that he had been beating his slave and he had been saying:I seek refuge with Allah, but he continued beating him, whereupon he said: I seek refuge with Allah's Messenger, and he spared him. Thereupon Allah's Messenger (ﷺ) said: By Allah, God has more dominance over you than you have over him (the slave). He said that he set him free
Reference: Muslim 4309