In 'Balance of Terror' when McCoy is in Kirk's quarters giving him an answer to Kirk's rhetorical question, ending with, "Don't give up on the one named 'Kirk'."
Also, in 'A Taste of Armageddon' when Kirk agrees with Anan 7 that humans are killers. "We can admit we're killers, but we're not going to kill today. And that's all it takes, knowing that we're not going to kill. Today."