Rank of derivative polynomial map equals dimension image?

Question

I've been told that given a polynomial map $f:X\to Y$ in characteristic zero, there exists an open dense subset $U$ of $X$ such that for all points $x$ in $U$, the rank of the derivative of $f$ in $x$ equals the dimension of the image of $f$. Could someone please explain to me why this is true, or direct me to some reference where this is explained?