No, the phrase "let sleeping dogs lie" is not attributed to William Pitt. It is an English idiom that means it's best not to disturb a situation as it may cause trouble or complications.
Let Sleeping Dogs... was created in 2005.
Let sleeping dogs lie means leave things as they are and do not stir up trouble.
Men Women and Dogs - 2001 Let Sleeping Dogs Lie 1-5 was released on: USA: 11 November 2001
Leave things as they are.
The Joker Blogs - 2008 Let Sleeping Dogs Lyle - 1.11 was released on: USA: 31 October 2009
No. Let sleeping dogs lie.
Road Rovers - 1996 Let Sleeping Dogs Lie 1-5 was released on: USA: 26 October 1996
Robert Walpole
It means don't stir up trouble.
The phrase is "let sleeping dogs lie." It is the same as "if it isn't broken, don't fix it." These mean something to the effect of if something is serving its purpose, don't change it. If you wake up the sleeping dog, it might bite you. If you fix something that isn't broken, it might not work the same.
The idiom "let sleeping dogs lie" means to avoid interfering in a situation that is currently stable, as doing so may cause trouble or disturbance. It advises against stirring up old problems or issues that are best left alone.
The cast of Let Sleeping Dogs Lie - 1995 includes: Geoff Lawson as First Terrorist Anthony Ofoegbu as First Detective Rocky Rising as Second Detective Aiden Waters as Michael Sabra Williams as Jo