Let $R$ be a ring, $A$ a (not necessarily commutative) $R$-algebra and $M$ a (left) A-module which is free of finite rank as an $R$-module. If $a\in A$ then multiplication with $a$ on $M$ is an $R$-linear endomorphism of $M$ and as such it has a characteristic polynomial $\chi_a$. I've learned from notes by Bart de Smit which I've accidentally found via Google (http://www.math.leidenuniv.nl/~desmit/notes/charpols.pdf) that the function $\chi_\bullet$ is determined by its values on a generating set of $A$ as an $R$-module. He proves this by deriving suitable formulas for the characteristic polynomial of a sum of two endomorphisms.
I'm looking for a reference for this fact that can be cited more easily than the informal notes above.