Mencius - Book 1, Part 1, Chapter 7B

The king asked again, 'Is such an one as I competent to love and protect the people?' Mencius said, 'Yes.' 'How do you know that I am competent for that?' 'I heard the following incident from Hû Ho: "The king," said he, "was sitting aloft in the hall, when a man appeared, leading an ox past the lower part of it. The king saw him, and asked, Where is the ox going? The man replied, We are going to consecrate a bell with its blood. The king said, Let it go. I cannot bear its frightened appearance, as if it were an innocent person going to the place of death. The man answered, Shall we then omit the consecration of the bell? The king said, How can that be omitted? Change it for a sheep." I do not know whether this incident really occurred.'

The king replied, 'It did,' and then Mencius said, 'The heart seen in this is sufficient to carry you to the royal sway. The people all supposed that your Majesty grudged the animal, but your servant knows surely, that it was your Majesty's not being able to bear the sight, which made you do as you did.'

The king said, 'You are right. And yet there really was an appearance of what the people condemned. But though Chî be a small and narrow State, how should I grudge one ox? Indeed it was because I could not bear its frightened appearance, as if it were an innocent person going to the place of death, that therefore I changed it for a sheep.'
~ James Legge translation via nothingistic.org ~

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