There is a fairly common theorem, which states that:
The sum of $n$ independent variables following an exponential distribution $\mathrm{Exp}(\alpha)$ follow an gamma distribution $\mathrm{Gamma} (n, 1/\alpha)$ (also known as Erlang distribution).
I'm using this theorem in my thesis. I've been asked to cite the paper that introduces, or, if it's not possible, to at least cite a paper that explicitly mentions it.
Anyone knows of any such papers?