From Sahih Bukhari (Hadith 30):
Narrated Al-Ma'rur: At Ar-Rabadha I met Abu Dhar who was wearing a cloak, and his slave, too, was wearing a similar one. I asked about the reason for it. He replied, "I abused a person by calling his mother with bad names." The Prophet said to me, 'O Abu Dhar! Did you abuse him by calling his mother with bad names You still have some characteristics of ignorance. Your slaves are your brothers and Allah has put them under your command. So whoever has a brother under his command should feed him of what he eats and dress him of what he wears. Do not ask them (slaves) to do things beyond their capacity (power) and if you do so, then help them
Reference: Bukhari 30