To be skeptical about a statement made, to take it with a grain of salt.
with a grain of salt [= mod.L. cum grano salis]: (to accept a statement) with a certain amount of reserve. Also in similar phrases, now esp. with a pinch of salt.
1647 TRAPP Comm. Rev. vi. 11 This is to be taken with a grain of salt. (That’s the earliest instance given in the OED).