  An optimally doped iron-based superconductor LaFeAsO$_{0.92}$F$_{0.08}$ with
$T_c = 29$ K has been studied by inelastic powder neutron scattering. The
magnetic excitation at $Q=1.15$ \AA$^{-1}$ is enhanced below $T_c$, leading to
a peak at $E_{res}\sim13$ meV as the resonance mode, in addition to the
formation of a gap at low energy below the crossover energy $\Delta_{c}\sim10
meV$. The peak energy at $Q=1.15$ \AA$^{-1}$ corresponds to $5.2 k_B T_c$ in
good agreement with the other values of resonance mode observed in the various
iron-based superconductors, even in the high-$T_c$ cuprates. Although the
phonon density of states has a peak at the same energy as the resonance mode in
the present superconductor, the $Q$-dependence is consistent with the resonance
being of predominately magnetic origin.
