This is an ancient adage, originally voiced by Kahless himself.

Interestingly, when the Federation first started learning about Klingons, Kahless was considered to be no more than an evil conqueror. He had formed an empire of, in James Kirk's words, "conquered worlds." The Klingons, Kirk felt, "take what they want by arms and force." As part of their examination of the behavior of other life forms, the Excalbians staged a conflict between the forces of "good"---represented by Kirk, Spock, and the images of Abraham Lincoln and the Vulcan philosopher Surak---and those of "evil"---represented by, among others, the image of Kahless. The Excalbians formed their replica of Kahless on the basis of the Federation's concept of him. Therefore, they described him in terms fitting an oppressor: "the Klingon who set the pattern for his planet's tyrannies."

In time, and as more was learned about Klingon history and culture, the Federation's understanding of the role of Kahless in Klingon history changed. It is now known that Klingons consider him a great warrior who did indeed conquer other worlds, but united the Empire by giving the people the laws of honor which direct every Klingon's life.