Islam has never believed in slavery! Islam accepted the slavery as a fact prevailing in society, which was rooted in pre-Islamic Age of Ignorance. Islam accepted it and tried to dissolve it by encouraging Muslims to release slaves in various occasions with this hope that slavery is gradually abolished and we see that the slavery was abolished all over the Muslim world.
Today, slavery is abolished in the Muslim world generally and among the Shiâa communities particularly. Some rare cases, if reported in some Muslim countries, are not approved by great Muslim authorities; and they are related to deviated groups.