I need a maximum principle in unbounded domains: if $u$ is a solution, bounded in $\Omega$, satisfying
$\Delta u+c(x)u=0, \ \ in \ \Omega,$ $c\in L^\infty$, $u\leq0 \ \ in \ \Omega$ $u(x_0)=0, \ \ x_0\in\Omega$ Then $u\equiv0 \ \ in \ \Omega$ Someone know where I can find this statement?