Can anyone provide me with the reference for the following fact (idea of the proof will be appreciated too):

Cohomology ring with $\mathbb Q$-coefficients of a group $G$ (I don't know precisely what the assumptions are: reductive complex algebraic group or maybe complex Lie group G with some restrictions. The cases I'm interested in are $GL_n(\mathbb C)$ and $SL_n(\mathbb C)$) is the exterior algebra on the generators of odd degrees, with the number of generators equal to the rank of $G$.

This fact is attributed to H.Hopf, but I wasn't able to find a reference.

Thanks.