# Eisenstein series and Hecke operators

I came across the following statement several times while looking through literature on Eisenstein series and Hecke operators (like Iwaniec's books, articles by Cogdell, etc). I am looking for a reference that includes a proof.

The Eisenstein series for $\Gamma_0(N)$ (at any cusp) are eigenfunctions for the Hecke operators $T_n$ with $(n,N)=1$.