Divergence theorem of product

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Len $\Omega$ be bounded domain in $\mathbb{R}^3$ with smooth boundary, $f$ smooth vector field on $\Omega$ with $f=0$ on $\partial \Omega$. How to use divergence theorem for $\displaystyle\int_{\Omega} (\textrm{div}\ f)^2 d^3\textbf{r}$ ? Thanks