Ammonia gas dissolves in water to form ${\mathrm{NH}}_4\mathrm{OH}$ In this reaction water acts as

$\underset{\mathrm{Acid}}{{\mathrm{H}}_2\mathrm{O}} + \underset{\mathrm{base}}{{\mathrm{NH}}_3} \rightleftharpoons {\mathrm{NH}}_4^{+} + {\mathrm{OH}}^{-}$

In this reaction ${\mathrm{H}}_2\mathrm{O}$ acts as acid because it donates a proton.