fluid dynamics - How does one derive the equation for the speed of sound?

In my acoustics books I see

$$c^2 = \frac{\mathrm{d}P}{\mathrm{d}\rho}$$

where $c$ is the speed of sound, $P$ is the pressure and $\rho$ is the density. Where does this equation come from? In my books it appears almost as a definition. Could you explain this, or at least point me to an article or a book that addresses this question? Thanks.