Matthew 17:18

16So I brought him to your disciples, and they could not cure him.”

17Jesus answered, “Faithless and perverse generation! How long will I be with you? How long will I bear with you? Bring him here to me.”

18Jesus rebuked the demon, and it went out of him, and the boy was cured from that hour.

19Then the disciples came to Jesus privately, and said, “Why weren’t we able to cast it out?”

20He said to them, “Because of your unbelief. For most certainly I tell you, if you have faith as a grain of mustard seed, you will tell this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible for you.