Leo Tolstoy 
Three Questions and Other Tales 

It once occurred to a King that if he knew the right moment when to begin on any work and the right kind of people to have or not to have dealings with and the thing to do that was more important than any other thing, he would always be successful. And he proclaimed throughout his kingdom that he would give a great reward to any one who could tell him what was the right moment for any action, and who were the most essential of all people, and what was the most essential thing of all to do.