Here is the reference trail, according to this <A HREF="http://www.jstor.org/stable/1971419">source</A>:

> Borel made the key observation [1] that the cohomology of the fixed
> point set was closely related to a torsion-free quotient. In the
> 1960’s, this was formalized as the “localization theorem” of
> Borel-Atiyah-Segal-Quillen [2,3].

 1. A. Borel, <A HREF="http://www.indiana.edu/~jfdavis/seminar/Borel,Seminar_on_Transformation_Groups.pdf">Seminar on transformation groups,</A> Annals of Math. Studies
    46, Princeton (1960) [Sect. XII Theorem 3.4]
    
 2. M.F. Atiyah and G. Segal, Equivariant cohomology and localization,
    lecture notes, 1965, Warwick.
    
 3. D. Quillen, <A HREF="http://www.jstor.org/stable/1970770">The
    Spectrum of an Equivariant Cohomology Ring I</A>, Annals of
    Mathematics **94**, 549–572 (1971). [Theorem 4.4].

----

A more extensive overview of the literature leading up to, and following after this localization theorem can be found <A HREF="http://www.math.ias.edu/~goresky/pdf/equivariant.jour.pdf">here</A> (section 1.7). In addition to the unpublished 1965 lecture notes of Atiyah and Segal, there is a 1968 publication [4], crediting the theorem to Segal [5].

  4. M.F. Atiyah and G.B Segal: <A HREF="http://folk.uio.no/rognes/suprema/trace/index-II.pdf">Index of elliptic operators II,</A> Ann. Math. **87**, 531–545 (1968).

  5. G.B. Segal, <A HREF="http://link.springer.com/article/10.1007%2FBF02684593?LI=true">Equivariant K-theory,</A> Publ. Math. Inst. Hautes Etudes (Paris) **34**, 129-151 (1968).