Matthew 17:14-20 (ESV)

And when they came to the crowd, a man came up to him and, kneeling before him, said, “Lord, have mercy on my son, for he is an epileptic and he suffers terribly. For often he falls into the fire, and often into the water. And I brought him to your disciples, and they could not heal him.” And Jesus answered, “O faithless and twisted generation, how long am I to be with you? How long am I to bear with you? Bring him here to me.” And Jesus rebuked the demon, and it came out of him, and the boy was healed instantly. Then the disciples came to Jesus privately and said, “Why could we not cast it out?” He said to them, “Because of your little faith. For truly, I say to you, if you have faith like a grain of mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move, and nothing will be impossible for you.”