Mencius - Book 5, Part 1, Chapter 6B

'That Tan-chû was not equal to his father, and Shun's son not equal to his; that Shun assisted Yâo, and Yü assisted Shun, for many years, conferring benefits on the people for a long time; that thus the length of time during which Shun, Yü, and Yî assisted in the government was so different; that Ch'î was able, as a man of talents and virtue, reverently to pursue the same course as Yü; that Yî assisted Yü only for a few years, and had not long conferred benefits on the people; that the periods of service of the three were so different; and that the sons were one superior, and the other superior: all this was from Heaven, and what could not be brought about by man. That which is done without man's doing is from Heaven. That which happens without man's causing is from the ordinance of Heaven.

'In the case of a private individual obtaining the throne, there must be in him virtue equal to that of Shun or Yü; and moreover there must be the presenting of him to Heaven by the preceding sovereign. It was on this account that Confucius did not obtain the throne.
~ James Legge translation via nothingistic.org ~

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