Matthew 17:15-20 New American Standard Bible (NASB)
15 “[a]Lord, have mercy on my son, for he is a [b]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 [c]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 [d]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.