The highest power of a prime $p$ that divides $n!$ is give by $p^{\left(\sum_{i=1}^\infty\left\lfloor\frac{n}{p^i}\right\rfloor\right)}$
I am fully aware of how to prove this but I don't have space to prove it in my dissertation. Therefore I need to reference it but I can't find it anywhere. I thought it would be easy to find as it's such a well known fact. Can anyone help?