A mixture of benzaldehyde and formaldehyde on heating with aqueous NaOH solution gives

Both compounds do not contain $\mathrm{\alpha }$-hydrogen hence undergo Crossed Cannizzaro reaction.
Initially OH^– attacks at the carbonyl carbon of HCHO than that of PhCHO because carbonyl carbon of HCHO is
(i) more electrophilic
(ii) less sterically hindered to give hydroxyalkoxide which acts as hydride donor in next step to give sodium formate.

${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CHO}+\mathrm{HCHO}\stackrel{\mathrm{NaOH}}{\to }\mathrm{HCOONa}+{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{CH}}_2\mathrm{OH}$