Mencius - Book 1, Part 2, Chapter 8

The king Hsüan of Ch'î asked, saying, 'Was it so, that T'ang banished Chieh, and that king Wû smote Châu?' Mencius replied, 'It is so in the records.'

The king said, 'May a minister then put his sovereign to death?'

Mencius said, 'He who outrages the benevolence proper to his nature, is called a robber; he who outrages righteousness, is called a ruffian. The robber and ruffian we call a mere fellow. I have heard of the cutting off of the fellow Châu, but I have not heard of the putting a sovereign to death, in his case.'
~ James Legge translation via nothingistic.org ~

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