I am searching for a reference for this apparently well known fact (the part below Theorem 1.1 in the picture i.e. the equation $(6)$):
This screenshot is from https://math.aalto.fi/~astalak2/files/pucci_conjecture.pdf. Here the operator $\mathcal{L}_Au=\text{Tr}(A(x)D^2u)$ (this can be found on the linked paper before Theorem 1.1). I am interested on this refence since I need this kind of maximum principle for the Poisson equation $\Delta u=f$ where the solution $u$ is in $W^{2,p}(\Omega)$ and the function $f\in L^p(\Omega)$, $p>n/2$.
Thus if anyone would know a refence, it would be greatly appreciated!