Mencius. Book 1, Part 2, Chapter 8.
1. The king Seuen of Ts'e asked, saying, "Was it so, that T'ang banished Kee, and that King Woo smote Chow?" Mencius replied, "It is so in the records."
2. The king said, "May a minister then put his sovereign to death?"
3. 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 Chow, but I have not heard of the putting a sovereign to death, in his case."
Text source: The Works of Mencius. Translated by James Legge.