Avoid citing sources in your abstract. There are two reasons for this:
The abstract should focus on your original research, not on the work of others.
The abstract should be self-contained and fully understandable without reference to other sources.
There are some circumstances where you might need to mention other sources in an abstract: for example, if your research responds directly to another study or focuses on the work of a single theorist. In general, though, don’t include citations unless absolutely necessary.