Matthew 17:17-21

And Jesus answered and said, O faithless and perverse generation, how long shall I be with you? how long shall I bear with you? bring him hither to me. And Jesus rebuked him; and the demon went out of him: and the boy was cured from that hour. Then came the disciples to Jesus apart, and said, Why could not we cast it out? And he saith unto them, Because of your little faith: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you.