Then the disciples came to Jesus privately and said, "Why could we not cast it out?"
Matthew 17:19
Surrounding context
17:18 And Jesus rebuked the demon, and it came out of him; and the child was cured from that very hour.
17:19 Then the disciples came to Jesus privately and said, "Why could we not cast it out?"
17:20 So Jesus said to them, "Because of your 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.