Matthew 17:19 in the King James Version
Then came the disciples to Jesus apart, and said, Why could not we cast him out?
Context
18 Jesus rebuked the demon, and it went out of him, and the boy was cured from that hour.
19 Then the disciples came to Jesus privately, and said, “Why weren’t we able to cast it out?”
20 He 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.