quantum chemistry - Why does the two-electron exchange integral in Hartree-Fock theory have positive values?

In Section 2.3.6 of Szabo & Ostlund's Modern Quantum Chemistry, the exchange integral has the form of

$$\int \mathrm{d}\mathbf{r}_1\,\mathrm{d}\mathbf{r}_2\, \psi_a^*(\mathbf{r}_1) \psi_b(\mathbf{r}_1)\frac{1}{r_{12}} \psi^*_a(\mathbf{r}_2) \psi_b(\mathbf{r}_2)$$

or $\langle ab|ba\rangle$ in physicists' notation. According to this textbook (and many other books), $\langle ab|ba\rangle$ is positive. The conclusion seems obvious but I just cannot find a proof. Is there any simple reason that $\langle ab|ba\rangle$ must be positive?