Residue of Rankin-Selberg Dirichlet series

Question

Let $f\in S_k(N,\chi)$ be a cusp forms, and let $R_f(s)=\sum_{n=1}^{\infty}\frac{a(n)^2}{n^s}$ the Rankin-Selberg Dirichlet series then $R_f(s)$ hase a pole at $s=k.$ Can someone suggest to me a reference in which I can find a proof of this fact.

Thanks!

Answer 1

Try section 2 of Ogg, "On a convolution of L-series", Invent. Math. 7 (1969).