Define the Legendre symbol $\left(\frac{a}{p}\right)$. Prove that, if $p$ is an odd prime, then

$$\left(\frac{2}{p}\right)=(-1)^{\frac{p^{2}-1}{8}} .$$

Use the law of quadratic reciprocity to calculate $\left(\frac{91}{167}\right)$.

[You may use the Gauss Lemma without proof.]