Here is the reference trail, according to this source:
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].
A. Borel, Seminar on transformation groups, Annals of Math. Studies 46, Princeton (1960).
M.F. Atiyah and G. Segal, Equivariant cohomology and localization, lecture notes, 1965, Warwick.
D. Quillen, The Spectrum of an Equivariant Cohomology Ring I, Annals of Mathematics 94, 549–572 (1971). [Theorem 4.4].
