  We report ^{75}As nuclear magnetic resonance studies on an underdoped
single-crystal Ba0.77K0.23Fe2As2 with Tc = 16.5 K. Below T_N = 46 K, the NMR
peaks for H//c split and those for H //a shift to higher frequencies, which
indicates that an internal magnetic field along the c axis develops below T_N.
The spin-lattice relaxation rate 1/T_1 measured at the shifted peak with H//a,
which experiences the internal field, shows a distinct decrease below
Tc({\mu}_0H = 12 T) = 16 K, following a T^3 relation at low temperatures. Our
results show unambiguously that antiferromagnetic order and superconductivity
coexist microscopically. The unusual superconducting state with the coexisting
magnetism is highlighted.
