And Jesus answering, saith unto them
To all the disciples; for what Peter said, he said in the name of them all; and according to Matthew, the disciples said, "how soon is the fig tree withered away?" To which this is an answer; though the Arabic version renders it, "to him"; as if the words were directed particularly to Peter:

have faith in God;
or "the faith of God", so the Vulgate Latin, Syriac, Persic, and Ethiopic versions; that is, exercise, and make use of that faith which has God for its author, which is the work of God, and of his operation, a free grace gift of his; and which has God for its object; and is supported by his power, and encouraged by his goodness, truth, and faithfulness: and so the Arabic version renders it, "believe in God"; not only that such things may be done, as the drying up a fig tree, but those that are much greater.

Taken from John Gill's Exposition of the Bible

Mark 11:22 In-Context

20 In the morning, as they went along, they saw the fig tree withered from the roots.

21 Peter remembered and said to Jesus, “Rabbi, look! The fig tree you cursed has withered!”

22 “Have faith in God,” Jesus answered.

23 “TrulyI tell you, if anyone says to this mountain, ‘Go, throw yourself into the sea,’ and does not doubt in their heart but believes that what they say will happen, it will be done for them.

24 Therefore I tell you, whatever you ask for in prayer, believe that you have received it, and it will be yours.