Matthew 17:18-20

18And Jesus rebuked the demon, and it came out of him; and the child was cured from that very hour.

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

20So Jesus said to them, “Because of your unbelief; for assuredly, I say to you, if you have faith as a mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible for you.