Matthew 17:19
Then the disciples of Jesus coming privately, said, Why could not we cast it out?
Mark 4:10
But when they were alone, those who were about him with the twelve, asked him the meaning of the parable.
Mark 9:28
And when he was come into the house, the 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?