Lenstra's paper on elliptic curve factorization references a result of Deuring, which states (the precise statement is copied from this paper):
If $r$ is an integer such that $\vert r\vert<2\sqrt{p}$, then the number of isomorphism classes over $\mathbb{F}_p$ with exactly $p+1-r$ points is equal to the number of equivalence classes of binary quadratic forms with discriminant $r^2-4p$.
Is there an English translation of Duering's original paper anywhere, or does anyone know of a proof in English?