14 And when they were 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 lunatick, and sore vexed: for ofttimes he falleth into the fire, and oft into the water. 16 And I brought him to thy disciples, and they could not cure him. 17 Then Jesus answered and said, O faithless and perverse generation, how long shall I be with you? how long shall I suffer you? bring him hither to me. 18 And Jesus rebuked the devil; and he departed out of him: and the child was cured from that very hour.
19 Then came the disciples to Jesus apart, and said, Why could not we cast him out? 20 And Jesus said unto them, Because of your unbelief: 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. 21 Howbeit this kind goeth not out but by prayer and fasting. - Mark 9:14-29 KJV

14 When they came to the crowd, a man came up to Jesus, falling on his knees before Him and saying, 15 "Lord, have mercy on my son, for he is a lunatic and is very ill; for he often falls into the fire and often into the water. 16 "I brought him to Your disciples, and they could not cure him." 17 And Jesus answered and said, "You unbelieving and perverted generation, how long shall I be with you? How long shall I put up with you? Bring him here to Me." 18 And Jesus rebuked him, and the demon came out of him, and the boy was cured at once. 19 Then the disciples came to Jesus privately and said, "Why could we not drive it out?" 20 And He said to them, "Because of the littleness of your faith; for truly I say to you, if you have faith the size of a mustard seed, you will say to this mountain, 'Move from here to there,' and it will move; and nothing will be impossible to you. 21 ["But this kind does not go out except by prayer and fasting."] - Matthew 17: 14-21 NASB