Reference request: Density of testfunctions in sobolev space $ W^{1}_{0}$

Question

can someone help me out and name a good source where the following statement ist proven?

$C^\infty_0(\Omega)$ is dense in $W^{1,p}_0(\Omega)$ with $\Omega \subset \mathbb{R}^n$ being an open, bounded domain with continuous boundary $\partial \Omega$.