# What is the source of Hopf's (boundary) lemma?

In an introductory course on PDE's I got as a project to prove and present a version of Hopf's (boundary) lemma. Namely:

Let $$\Omega \subset R^{d}$$ be an non-empty open connected set with a twice differentiable boundary. Let $$u\in C^{2}(\Omega) \cap > C^{1}(\bar{\Omega})$$ be a subharmonic function ($$\Delta u \geq 0)$$. If there exists a $$M\geq 0$$ such that $$u(x) < M$$ for all $$x\in \Omega$$ and $$u(x_{0}) = M$$ for some $$x_{0}$$ on the boundary of $$\Omega$$ then the outward normal derivative at $$x_{0}$$ $$(\nabla u(x_{0})\cdot n)$$ is stricly positive.

Where can I find the original source of Hopf's lemma? I have looked trough Evans - Partial Differential Equations (second edition) and some notes written by professor John K. Hunter at UC Davis, but have been unable to make any progress.