Matthew 17:14-20 New International Version (NIV)
14 When they came to the crowd, a man approached Jesus and knelt before him. 15 “Lord, have mercy on my son,” he said. “He has seizures and is suffering greatly. He often falls into the fire or into the water. 16 I brought him to your disciples, but they could not heal him.
17 “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.” 18 Jesus rebuked the demon, and it came out of the boy, and he was healed at that moment.
19 Then the disciples came to Jesus in private and asked, “Why couldn’t we drive it out?”
20 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.”