Matthew 13:10
Then His disciples came up to Him and asked, "Why do you speak to them in stories?"
Mark 4:10
When He was by Himself, those who stayed about Him with the Twelve began to ask Him about the stories.
Mark 4:33-34
With many stories like these He kept on telling them the message, as far as they could understand it.
Treasury of Scripture Knowledge
Summary
And the disciples came, and said unto him, Why speakest thou unto them in parables?