From The Crito:
Soc. Are we to say that we are never intentionally to do wrong, or that in one way we ought and in another way we ought not to do wrong, or is doing wrong always evil and dishonorable, as I was just now saying, and as has been already acknowledged by us? Are all our former admissions which were made within a few days to be thrown away? And have we, at our age, been earnestly discoursing with one another all our life long only to discover that we are no better than children? Or are we to rest assured, in spite of the opinion of the many, and in spite of consequences whether better or worse, of the truth of what was then said, that injustice is always an evil and dishonor to him who acts unjustly? Shall we affirm that?
Soc. Then we must do no wrong?
Cr. Certainly not.
Soc. Nor when injured injure in return, as the many imagine; for we must injure no one at all?
Cr. Clearly not.
Soc. Again, Crito, may we do evil?
Cr. Surely not, Socrates.
Soc. And what of doing evil in return for evil, which is the morality of the many-is that just or not?
Cr. Not just.
Soc. For doing evil to another is the same as injuring him?
Cr. Very true.