Chemistry: The Central Science (13th Edition)

$H^+$: Lewis acid $N{H_2}^-$: Lewis base
The $N{H_2}^-$ has one pair of electrons free, so when it reacts with $HIO$, the $H^+$ will receive that pair. Since $N{H_2}^-$ donates one pair of electrons, it is the Lewis base. Since $H^+$ receive these electrons, it is the Lewis acid.