Ancient Caeser (roman) quote?

I need some help, I'm trying to remember an ancient roman quote from the emperor Caesar, wich was something about showing your true power by letting other people live, than actually kill them, it was something like that. I remember Schindler mentioned it, in the movie Schindler's List, to an SS Sargent (I think it was sargent? Maybe an officer? I don't know) who was his friend.