Units in $\mathbb{Z} [\text{some root of unity}]$

Question

The assertion is that for $\xi$ a primitive root of unity of degree $n$, then in $\mathbb{Z}[\xi]$, $\xi-1$ is a unit if $n$ is not a prime power.

It should be an easy proof, but I still don't have a good idea. Any hint is appreciated!