Mencius - Book 5, Part 1, Chapter 4A

Hsien-ch'iû Mang asked Mencius, saying, 'There is the saying, "A scholar of complete virtue may not be employed as a minister by his sovereign, nor treated as a son by his father. Shun stood with his face to the south, and Yâo, at the head of all the princes, appeared before him at court with his face to the north. Kû-sâu also did the same. When Shun saw Kû-sâu, his countenance became discomposed. Confucius said, At this time, in what a perilous condition was the kingdom! Its state was indeed unsettled." I do not know whether what is here said really took place.'

Mencius replied, 'No. These are not the words of a superior man. They are the sayings of an uncultivated person of the east of Ch'î. When Yâo was old, Shun was associated with him in the government. It is said in the Canon of Yâo, "After twenty and eight years, the Highly Meritorious one deceased. The people acted as if they were mourning for a father or mother for three years, and up to the borders of the four seas every sound of music was hushed." Confucius said, "There are not two suns in the sky, nor two sovereigns over the people." Shun having been sovereign, and, moreover, leading on all the princes to observe the three years' mourning for Yâo, there would have been in this case two sovereigns.'
~ James Legge translation via nothingistic.org ~

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