It is not entirely unreasonable to include a citation in an abstract, if the reason you are citing it is because your paper is a major extension, rebuttal, or counterpoint to the cited article.
In that case, however, you do have the responsibility of providing the reference within the body of the abstract. For example,
We extend upon the results of Smith [Journal of Very Important Results, 1, 374 (2012)] to include the effects of a doohickey at the end of the thingamajig.
In such a case, the abstract remains self-contained, with an important citation included. (This is especially essential if an author is well-known for multiple papers, in which case the reference can be used to distinguish the varous works that could be intended.)
Large numbers of citations, however, should be avoided, as should "secondary" citations. Only the most critical literature for a paper should be cited, and that should normally be limited to one or two. Any more than that, and the abstract becomes hard to read.