One article on elliptic curves over finite fields makes the following claim: "Let $E$ be an elliptic curve over $\mathbb{F}_q$. The rings End$_{\mathbb{F}_q}(E)$ and End$_{\overline{\mathbb{F}_q}}(E)$ are either complex quadratic orders or maximal orders in $\mathbb{Q}_{\infty,p}$" where $\mathbb{Q}_{\infty,p}$ is a quartenion algebra that ramifies only at $p$ and $\infty$.
Other article mentions that "It is known that End$(E)$ is an order in a finite-dimensional division algebra over $Q$".
Apparently these things are meant to be know, but i couldn't find any proof for these claims. Can someone explain any of those or provide me a source?