From the introduction of Ribet-Stein:

Shimura showed that if we start with the elliptic curve $E$ defined by the equation $y^2 +y = x^3 −x^2$ then for “most” $n$ the image of $\rho$ is all of $\mathrm{GL}_2(\mathbf{Z}/n\mathbf{Z})$.

Here $\rho$ is the representation of $\mathrm{Gal}(\overline{\mathbf{Q}}/\mathbf{Q})$ on the $n$-torsion of $E$. What is the original paper where Shimura shows this? (Is there an online copy?)