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 irreducible *smooth*  complex algebraic varieties (separated, reduced, irreducible schemes of finite type over $\Bbb C$) with *smooth*  fibres over closed points of $Y$. The $f$ is smooth if and only if all these fibres  have the same dimension $\dim X-\dim Y$.