Slavery existed before Muhammad’s time. In fact, it was part of many social systems in world. Slaves were considered as assets and part of people’s wealth. Since Islam protects peoples’ wealth and property, slavery was gradually abandoned.
In the early days of Islam, abusing and mistreating a slave was considered a sin that required repentance to God by releasing the slave free. In addition, repentance from violations of many Islamic rules required releasing slaves or buying a slave from someone and freeing him or her. This had continued until slavery was fully abolished.
Muhammad encouraged believers to release slaves for person called “Abu Mas’ud Al-Badri” hitting and lashing his slave. Muhammad said to him firmly:
You should know that God is more able and has more power over you than your power over this slave.” Abu Mas’ud cooled down and said to Muhammad in an apologetic language: “I will release him for the sake of God.” Muhammad said to him: “If you don't do that, the Hellfire will touch your face
(Sahih Muslim, 5/91, 1659)