Matthew 17:19
Then the disciples came to Jesus privately and said: Why were we not able to send it out?
Mark 4:10
When he was alone his followers including the twelve asked about the illustrations.
Mark 9:28
When the child went into the house his disciples asked him privately: Why were we unable to send it out?
Treasury of Scripture Knowledge
Summary
Then came the disciples to Jesus apart, and said, Why could not we cast him out?