Mencius - Book 2, Part 1, Chapter 9A

Mencius said, 'Po-î would not serve a prince whom he did not approve, nor associate with a friend whom he did not esteem. He would not stand in a bad prince's court, nor speak with a bad man. To stand in a bad prince's court, or to speak with a bad man, would have been to him the same as to sit with his court robes and court cap amid mire and ashes.

'Pursuing the examination of his dislike to what was evil, we find that he thought it necessary, if he happened to be standing with a villager whose cap was not rightly adjusted, to leave him with a high air, as if he were going to be defiled. Therefore, although some of the princes made application to him with very proper messages, he would not receive their gifts. He would not receive their gifts, counting it inconsistent with his purity to go to them.
~ James Legge translation via nothingistic.org ~

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