I am asking for a *reference* for the following lemma (for which I know a proof).

Lemma.Let $f\colon X\to Y$ be a surjective morphism of irreduciblesmoothcomplex algebraic varieties (separated, reduced, irreducible schemes of finite type over $\Bbb C$) withsmoothfibres over closed points of $Y$. The $f$ is smooth if and only if all these fibres have the same dimension $\dim X-\dim Y$.