  We used high-resolution scanning tunneling spectroscopy to study the
hole-doped iron pnictide superconductor Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$
($T_c=38$ K). Features of a bosonic excitation (mode) are observed in the
measured quasiparticle density of states. The bosonic features are intimately
associated with the superconducting order parameter and have a mode energy of
$\sim$14 meV, similar to the spin resonance measured by inelastic neutron
scattering. These results indicate a strong electron-spin excitation coupling
in iron pictnide superconductors, similar to that in high-$T_c$ copper oxide
superconductors.
