John Gill Commentary Matthew 16:8

"And Jesus perceiving it said, O ye of little faith, why reason ye among yourselves, because ye have no bread?" — Matthew 16:8 (ASV)

Which when Jesus perceived
Without hearing any of their debates, but by his omniscience; for he knew the doubts and unbelief, and anxious solicitude of their minds, as well as their private reasonings one with another:

he said unto them, O you of little faith ;
a phrase used upon a like occasion, when he would dissuade his disciples from an anxious distressing care about a livelihood, (Matthew 6:30) (See Gill on Matthew 6:30):

why reason you among yourselves, because you have brought no bread?
blaming one another for your negligence and forgetfulness in this matter; distressing your minds, as if you should be famished and starved, because you have not brought a quantity of bread, as you used to do with you.