Islam institutionalized slavery. Muhammad began to take slaves after he moved to Medina, and had power. Slaves were usually taken in raids on nearby Arab tribes, or war, either through offensive or defensive actions. Islam allows the taking of slaves as "booty", or reward for fighting. This has led to numerous "jihads" by Muslim states and tribes to attack other non-Muslim groups and obtain slaves. Islamic jurisprudence laid down regulations for the proper treatment of slaves. However, abuses have occurred throughout history.
INTRODUCTION
The West is familiar with the history of slavery in the new world. It was sinful and terrible, and it lasted for several hundred years. And it was abolished mainly through the efforts of Christians in England (Wilberforce, Clarkson) and America (the Abolitionists, primarily Protestant).
However, few people in the west know about Islam and slavery. Most would be surprised that Islam authorizes the taking of slaves as spoils of war. From the days that Muhammad drew his sword to rob and conquer non-Muslims to this very day, Muslims have been taking non-Muslims, and even other black Muslims, as slaves.
http://answering-islam.org.uk/Silas/slavery.htm