Matthew 17:19
Then the disciples came to Jesus apart, and said, Why could not we cast it out?
Mark 4:10
And when he was in private, they who were about him, with the twelve, asked him about the parables.
Mark 9:28
And when he had come into the house, his disciples asked him privately, Why could not we cast it out?
Treasury of Scripture Knowledge
Summary
Then came the disciples to Jesus apart, and said, Why could not we cast him out?