Matthew 17:14-21 New King James Version (NKJV)
14 And when they had come to the multitude, a man came to Him, kneeling down to Him and saying, 15 “Lord, have mercy on my son, for he is [a]an epileptic and suffers severely; for he often falls into the fire and often into the water. 16 So I brought him to Your disciples, but they could not cure him.”
17 Then Jesus answered and said, “O [b]faithless and perverse generation, how long shall I be with you? How long shall I bear with you? Bring him here to Me.” 18 And Jesus rebuked the demon, and it came out of him; and the child was cured from that very hour.
19 Then the disciples came to Jesus privately and said, “Why could we not cast it out?”
20 So Jesus said to them, “Because of your [c]unbelief; for assuredly, I say to you, if you have faith as a mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible for you. 21 [d]However, this kind does not go out except by prayer and fasting.”