Congruence condition for minus one to be a quadratic residue

==Statement

Suppose $p$ is an odd prime. Then, $-1$ is a quadratic residue modulo $p$ if and only if $p\equiv 1{\pmod {4}}$. Equivalently, $-1$ is a quadratic nonresidue modulo $p$ if and only if $p\equiv 3{\pmod {4}}$.