Matthew 17:14-21

14And when they had come to the multitude, there came to him a certain man, kneeling down to him, and saying, 15Lord, have mercy on my son: for he is lunatic, and greatly distressed: for often he falleth into the fire, and often into the water. 16And I brought him to thy disciples, and they could not cure him. 17Then 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 here to me. 18And Jesus rebuked the demon; and he departed out of him: and the child was cured from that very hour. 19Then came the disciples to Jesus privately, and said, Why could not we cast him out? 20And Jesus said to them, Because of your unbelief: for verily I say to you, If ye have faith as a grain of mustard seed, ye shall say to this mountain, Move from here to yonder place; and it shall move; and nothing shall be impossible to you. 21However this kind goeth not out except by prayer and fasting.