In Cannizzaro reaction given below: 

$2\mathrm{PhCHO}\stackrel{-\mathrm{OH}}{⟶}{\mathrm{PhCH}}_2\mathrm{OH}+{\mathrm{PhCO}}_2^{-}$ 

The slowest step is

The attack of ${ }^{-}\mathrm{OH}$ at the carboxylic group

The deprotonation of ${\mathrm{PhCH}}_2\mathrm{OH}$

The abstraction of proton from the carboxylic group

The transfer of hydride to the carbonyl group