An undirected graph may be regarded as a resistor network where each edge corresponds to a resistor of unit resistance. This paper covers such an approach.

*On electric resistances for distance-regular graphs*by Jack Koolen, Greg Markowsky, and Jongyook Park

I paraphrase some statements from Chapter 3 of this paper.

Consider a graph $G$ as a resistor network where each edge is a unit resistance.
To compute the effective resistance between two vertices $u$ and $v$, assume that a battery of voltage $V$ is attached to $u$ and $v$ producing a current through the graph.
The current flowing through the circuit can be determined by calculating the voltage at each point on the graph.
A function $f$ on the vertex set $V$ of $G$ is **harmonic** at a point $z\in V$ if $f(z)$ is the average of neighboring values of f; that is,
\begin{equation*}
\sum_{\text{for any $x$ adjacent to $z$}}\big(f(x)-f(z)\big) = 0.
\end{equation*}
The voltage function on V can be characterized as the *unique* function which is harmonic on $V − \{u, v\}$ having the prescribed values on $u$ and $v$.

I'd like to use this characterization on my research. But I cannot find a reference for this 'harmonic characterization' on any other textbook. Even the above paper doesn't cite any reference for this statement. I skim through many graph theory textbooks (such as Bollobas) but in vain.

Is this characterization true? Which textbooks or papers can I use as a reference?