I search an reference where wi found the following generalized maximum principle
let $G$ be a bounded domain, $u$ be a positive solution of the Dirichlet problem for the Poisson equation $\Delta u =f$ in $G$ (so that u is continuous in $\bar G$ and $u=0$ at the boundary), $f$ in $L^2(G)$ then $$ \max_{x\in G} u(x) \leq K \Vert f\Vert_{L^1(G)} (\operatorname{mes} G)^{1/n} $$ where constant $K$ depends on the dimension $n$ only.
Thank you in advance for the help