Matthew 21:20

And when the disciples saw it
The next day in the morning, as Mark says: they had, heard what Christ had said to it the day before, as the same evangelist observes; but did not take notice of the immediate withering of the tree; but the next morning, as they returned from Bethany, they saw it dried up from the roots:

they marvelled;
not that Christ should curse it, but that it should wither away so soon, and upon his saying what he did; which was a considerable instance of his power and Godhead, all creatures, animate and inanimate, being at his command and disposal:

saying, how soon is the fig tree withered away?
This was said by Peter, in the name of the rest, who recollecting what Jesus had said to it the day before, and observing how the event had answered his words so soon, addressed Christ after this manner: "master, behold the fig tree which thou cursedst is withered away"; expressing his wonder at it, and ascribing, it to the power of Christ; of which this was an amazing proof and evidence.