  We report an $^{75}$As NMR study of BaFe$_2$As$_2$ in both single crystals
and polycrystal forms. We find that Sn impurities in the single crystal
dramatically alter the low energy spin fluctuations and suppress the ordering
temperature from 138 K to 85 K. In contrast to Sn-free samples, we find that
the temperature dependence of the $^{75}$As NMR spectra and spin lattice
relaxation rates reveal a second order phase transition to a state of
incommensurate magnetic order.
