Mark 9:28

And when he was come into the house, his disciples asked him privately, Why could not we cast him out?

Cross-reference

Mark 9:18 describes the father's report of the disciples' failure to cast out the demon, which prompts this private question.

Matthew 17:19 is the parallel account of the same event: disciples ask privately why they couldn't cast out the demon.

Matthew 17:20 contains Jesus' response to that question, directly continuing the same conversation.