Albert Barnes Commentary Matthew 21:20

"And when the disciples saw it, they marvelled, saying, How did the fig tree immediately wither away?" — Matthew 21:20 (ASV)

And when the disciples saw it. That is, on the morning following that on which it was cursed (Mark 11:20).

They marvelled, saying, etc. Peter said this (Mark 11:21). Matthew means only to say that this was said to him; Mark tells us which one of them said it.