What to do when my introductory sentence is taken from another author's paper? I am about ready to sumbit a paper in mathematics, but I have one worry.  While researching, I came across a sentence in the introduction of an old paper that I really want to use (with a very minor modification) in my abstract.  It has been in all of my drafts for a long while now, and I can't see getting rid of it!!!

Is this a serious issue?  Even though I'm stealing the sentence, I am not actually stealing any mathematics, so it kind of seems (??) okay to me. The old paper is cited elsewhere in my paper, by the way.  I just don't know how I can give credit for a phrase when it appears in the abstract... it would seem overkill to put the whole citation in the abstract. 

In times like these I wish I hadn't ditched all my English major friends. 

EDIT: THANKS for all of the replies. You convinced me to change to wording, and I believe I ended up with something almost as good.  I am a nervous wreck as it is, so I definitely don't need this plagiarism thing weighing on my conscience!!
I think some of the early advice on this question was really bad. It's easy to give casual attribution for a quote if you do it like this:


  Abstract : This paper is devoted to the study of K-algebras. It has
  long been known that 'All regular K-algebras are equivalent, but every
  irregular K-algebra is irregular in its own way.' (Tolstoy). We
  classify a new family of irregular K-algebras. This result has
  implications in algebra theory.


Then you can cite the paper properly in the body of your paper. Writing the abstract like this makes it very clear who the quote came from and you can deal with the where later.