  We use $^{75}$As nuclear magnetic resonance (NMR) to investigate the local
electronic properties of Ba(Fe$_{1-x}$Ru$_{x}$)$_2$As$_2$ ($x =$ 0.23). We find
two phase transitions, to antiferromagnetism at $T_N \approx$ 60 K and to
superconductivity at $T_C \approx$ 15 K. Below $T_N$, our data show that the
system is fully magnetic, with a commensurate antiferromagnetic structure and a
moment of 0.4 $\mu_B$/Fe. The spin-lattice relaxation rate $1/^{75}T_1$ is
large in the magnetic state, indicating a high density of itinerant electrons
induced by Ru doping. On cooling below $T_C$, $1/^{75}T_1$ on the magnetic
sites falls sharply, providing unambiguous evidence for the microscopic
coexistence of antiferromagnetism and superconductivity.
