Does any one know how to write the Maxwell equations as an equation on a principle $U(1)$-bundle?

In $\textit{Freed & Uhlenbeck: Instantons and Four manifolds}$, the author claims that the Maxwell equations can be written in a gauge theoretic way: the electro-magnetic field can be viewed as the curvature $F$ of a $U(1)$-bundle on the 4-dimensional Lorentz manifold, and the Maxwell equations are equivalent to $d^∗_A(F)=0$, where A is the connection. Does anyone have a reference for this statement? I just want to have a look at the proof and see how everything matches.