Matthew 17:18-20

18Jesus rebuked the demon, and it came out of him, and the boy was healed at once.

19Then the disciples came to Jesus privately and asked, “Why could we not drive it out?”

20He answered, “Because of your little faith [your lack of trust and confidence in the power of God]; for I assure you and most solemnly say to you, if you have [living] faith the size of a mustard seed, you will say to this mountain, ‘Move from here to there,’ and [if it is God’s will] it will move; and nothing will be impossible for you.