Sahih Muslim Book 015, Hadith Number 4089.
Chapter : How should the masters treat their slaves and expiation if they show high-handedness.
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 (may peace be upon him) said: By Allah, God has more dominance over you than you have over him (the slave). He said that he set him free. This hadith has been narrated on the authority of Shu'ba with the same chain of transmitters, but made no mention of (these words) of his: I seek refuge with Allah, I seek refuge with Allah's Messenger (may peace be upon him).