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