Mencius - Book 1, Part 2, Chapter 9

Mencius, having an interview with the king Hsüan of Ch'î, said to him, 'If you are going to build a large mansion, you will surely cause the Master of the workmen to look out for large trees, and when he has found such large trees, you will be glad, thinking that they will answer for the intended object. Should the workmen hew them so as to make them too small, then your Majesty will be angry, thinking that they will not answer for the purpose. Now, a man spends his youth in learning the principles of right government, and, being grown up to vigor, he wishes to put them in practice; if your Majesty says to him, "For the present put aside what you have learned, and follow me," what shall we say?

'Here now you have a gem unwrought, in the stone. Although it may be worth tails, you will surely employ a lapidary to cut and polish it. But when you come to the government of the State, then you say, "For the present put aside what you have learned, and follow me." How is it that you herein act so differently from your conduct in calling in the lapidary to cut the gem?'
~ James Legge translation via nothingistic.org ~

Go here to read the introductory post to this serialized version of the Works of Mencius.