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?