There is an estimation of a $ \sum_{x\in\mathbb{F}_p} \left(\frac{a_{n}x^n+a_{n-1}x^{n-1}+...a_0}{p} \right) \le (n-1)\sqrt{p}$

Question

Where can I find a proof of the following inequality? ( $n$ is odd)

$$ \sum_{x\in\mathbb{F}_p} \left(\frac{a_{n}x^n+a_{n-1}x^{n-1}+...a_0}{p} \right) \le (n-1)\sqrt{\vphantom{d}p} $$

I read that Helmut Hasse (for $n = 3$) and Andre Weil (for any $n$) proved that inequality, but I can't find anything.