I brought him to your disciples, but they could not heal him.”
“You unbelieving and perverse generation,” Jesus replied, “how long shall I stay with you? How long shall I put up with you? Bring the boy here to me.”
Jesus rebuked the demon, and it came out of the boy, and he was healed at that moment.
Then the disciples came to Jesus in private and asked, “Why couldn’t we drive it out?”
He replied, “Because you have so little faith. Truly I tell you, if you have faith as small as a mustard seed, you can say to this mountain, ‘Move from here to there,’ and it will move. Nothing will be impossible for you.”
References for Matthew 17:20