Euler's First Proof of Stirling's Formula

We present a proof given by Euler in his paper {\it ``De serierum determinatione seu nova methodus inveniendi terminos generales serierum"} \cite{E189} (E189:``On the determination of series or a new method of finding the general terms of series") for Stirling's formula. Euler's proof uses his theory of difference equations with constant coefficients. This theory outgrew from his earlier considerations on inhomogeneous differential equations with constant coefficients of finite order that he tried to extend to the case of infinite order.