Matthew 17:10-20 NIV
The disciples asked him, “Why then do the teachers of the law say that Elijah must come first?”
Jesus replied, “To be sure, Elijah comes and will restore all things. But I tell you, Elijah has already come, and they did not recognize him, but have done to him everything they wished. In the same way the Son of Man is going to suffer at their hands.” Then the disciples understood that he was talking to them about John the Baptist.
When they came to the crowd, a man approached Jesus and knelt before him. “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. 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.”
NIV: New International Version