It's from Statism and Anarchy. Here's the full quote:
"Ultimately, from whatever point of view we look at this question, we come always to the same sad conclusion, the rule of the great masses of the people by a privileged minority. The Marxists say that this minority will consist of workers. Yes, possibly of former workers, who, as soon as they become the rulers of the representatives of the people, will cease to be workers and will look down at the plain working masses from the governing heights of the State; they will no longer represent the people, but only themselves and their claims to rulership over the people. Those who doubt this know very little about human nature."