TKW p. 59:

quv Hutlh HoHbogh tlhIngan 'ach qabDaj 'angbe'bogh
The Klingon who kills without showing his face has no honor.

This is an adage about Klingon behavior cited by K'mpec, leader of the Klingon High Council, when he revealed to Picard that he had been poisoned. Worf took it one step further,  assuming that, since no Klingon would behave in such a dishonorable fashion, the assassination could not have been carried out by a Klingon. "A Klingon would not use poison," he stated. "The murder would have no honor."