A slogan I frequently hear is: "the Casson invariant is the Euler characteristic of the Floer homology of flat SU(2)-connections on the integral homology sphere". Is there a single paper/reference that essentially states this as a result? It has been difficult to locate precise definitions. In addition, a sketch of how this works would be very helpful. 



Casson invariant and Euler characteristic
Just to finalize comments since people are upvoting the question: The canonical reference is Taubes' *"Casson's invariant and gauge theory"* which makes the statement rigorous and has all the definitions (based on the relevant Chern-Simons functional). Floer's paper *"An instanton-invariant for 3-manifolds"* does build the Floer homology using this functional, and asserts that its Euler characteristic is (twice) the Casson invariant, but he only quotes Taubes' paper (that's what the reference [3] in Floer's paper really is).


Although I think Taubes' paper is beautiful enough that a sketch isn't needed, you can find a background sketch/summary in Saveliev's book (*"Invariants of homology 3-spheres"*), Chapter 5: *"Casson Invariant and Gauge Theory"*.


