What Is Dirac's Identity in Minkowski Spacetime?

[Abrain]

Do somebody knows anything about the Dirca's identity?
$$\begin{equation} \label{Dirac} \frac{\partial^2}{\partial x_{\mu}\partial x^{\mu}} \delta(xb_{\mu}xb^{\mu}) = -4\pi \delta(xb_0)\delta(xb_1)\delta(xb_2)\delta(xb_3) \end{equation}$$
here
$$xb$$, is the 4-vector $$x-b$$ in Minkowsky spacetime
$$\delta$$ is the Dirac delta function
$$x_0 = -x^0, \quad x_1 = x^1, \quad x_2 = x^2, \quad x_3, = x^3$$
Do you know where can i find some material about it?

Thanks!

 

[Abrain]

Nobody knows anything about this? [= UP]