What is a reference for the (classical and well-known) proof of Weyl's lemma that states:
Let $U$ be an open subset of $R^n$. Then if $f\in L^1_{loc} (U)$ and if $\int_U \hspace{1mm}{f\phi_\bar{z}}=0\;\;\;\forall \phi \in C_c^{\infty}(U) $, then $f$ is a.e. equal to a holomorphic function.
Just any quick and good reference would be appreciated. I know Weyl's lemma has a weaker form involving weak Laplacian. Where can I find a proof of that?