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 of the same dimension $\dim X-\dim Y$ over closed points of $Y$. Then the morphism $f$ is smooth.

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