In pokorski's "Gauge Field Theories" book, page 108 we find equation (2.87)

$$\int{}\mathcal{D}\phi{}e^{iS_0[\phi]+iS_I[\phi]+i\int{}d^4y\,\phi(y)J(y)}=e^{iS_I[-i\frac{\delta}{\delta{}J(x)}]}\int{}\mathcal{D}\phi{}e^{iS_0[\phi]+i\int{}d^4y\,\phi(y)J(y)}$$

where we are dealing with some (for our purposes unspecified) theory of a real scalar field $\phi$, where $S_0$ is the free part of the action, while $S_I$ is the interaction part; and $J$ is the current of that appears in the generating functional. Can anybody make explicit all the necessary steps to go from the left hand side to the right?