I need this to place in the list of references. ...
This is more like a question: Is a citation here really necessary? I mean, it appears to be a mathematical fact - you can directly evaluate:
Sum[Pochhammer[1 - b, k]/((a + k) k!), {k, 0, Infinity}]
to get
$$
\frac{\Gamma(1+a) \cdot \Gamma(b)}{a \cdot \Gamma(a+b)}
$$
and from this it is evident that we see here the Euler beta function.
But if you really need a citation - here is one from Wikipedia.
