I'm looking for a reference for the following result (all rings commutative with unit) :
Let $A$ be a ring, $B$ be an $A$-algebra which is free of finite rank as an $A$-module, $M$ be a free $B$-module of finite rank and $u$ an endormorphism of $M$. Then we have
$\det_A(u) = N_{B/A}(\det_B(u))$