Let $R=\mathbb{Q}[x_{i,j}\,:\, 1\leq i,j\leq n]$. Let $M$ be the $n\times n$ matrix $(x_{i.j})$. Let $\chi(M)$ be the characteristic polynomial of $M$. Finally, let $I$ be the ideal of $R$ generated by the non-constant coefficients of $\chi(M)$.
It should be well known that $I$ is a prime ideal of $R$. Is there a good reference for this fact?