Let $X$ be a regular Hunt process on $R^+$ with starting at $x$ and $T_y :=\inf\{t>0: X_t=y\}, y\in R^+$. $G_q(\cdot,\cdot), q >0$ denotes Green kernel of the process $X$. We have the following result:
$E_x (e^{-qT_y})=\frac{G_q(x,y)}{G_q(y,y)}.$
If you know which paper or book mentions about it, please tell me the references.
Thank you very much!