Reference request for equivalence of reducibility of $\ell$-adic Galois representation and isogeny of degree $\ell$ on elliptic curve

Question

Let $E$ be an elliptic curve over $\mathbb{Q}$. For each prime $\ell$, the action of $\mathrm{Gal}(\bar{\mathbb{Q}}/\mathbb{Q})$ on $E[\ell]$ (the group of $\ell$-division points of $E$) defines a representation $$\rho=\rho_\ell:\mathrm{Gal}(\bar{\mathbb{Q}}/\mathbb{Q}) \longrightarrow \mathrm{GL}(2,\mathbb{F}_\ell). $$

Where can I read the proof of:

$\rho$ is reducible if and only if $E$ admits an isogeny of degree $\ell$