Let sleeping dogs lie
Avoid interfering in a situation which is currently stable.
'Let sleeping dogs lie' derives from the long-standing observation that dogs are often unpredictable when they are suddenly disturbed. Geoffrey Chaucer was one of the first to put this notion into print, in Troilus and Criseyde, circa 1380, although the belief itself may well be much older:
"It is nought good a slepyng hound to wake."
The expression may have started as a warning about the risk of waking a potentially dangerous animal, but it later turned metaphorical. By the time it became established as a proverb its meaning had 'leave well alone', or as we might have it in the 21st century, "if it ain't broke don't fix it".
The cautionary phrase was well enough known by the 16th century for it to have been included as a proverb in John Heywood's definitive A Dialogue Prouerbes English Tongue, 1546:
It is euill wakyng of the slepyng dog.
At this point I ought to mention the 18th century British politician Sir Robert Walpole. 'Let sleeping dogs lie' is frequently associated with or even attributed to Walpole and the proverb is many times mentioned in print as being his motto. It is so closely associated with him as to have been the source of a later cartoon. The reason for that attribution is never explained and doesn't appear to be based on any evidence. The expression appears nowhere in any of Walpole's published writings and, as far as I can tell, there is no record of his ever uttering it. Of course, he may have repeated the form of the proverb printed earlier by Heywood. Sir Robert may have been in the habit of advising caution in political policy but, as far as the evidence shows, he didn't coin or even use the expression 'let sleeping dogs lie'.
To get to the current wording of the proverb we have to move to the 19th century. In December 1822 The London Magazine published a fanciful mariner's yarn entitled The Second Tale of Allan Lorburne, which included:
Let sleeping dogs lie, said the daft man, when he saw the dead hound before him.
The story is, to current sensibilities, pretty much bonkers. 'Daft' seems to be used to mean 'insane' and the 'hound' apparently alludes to a stricken sailing ship. Nevertheless, the citation does appear to be the first use of the proverb 'let sleeping dogs lie' in print.