let sleeping dogs lie

Cf. early 14th-cent. Fr. n'esveillez pas lou chien qui dort, wake not the sleeping dog.

It is nought good a slepyng hound to wake.
[c 1385 Chaucer Troilus & Criseyde iii. 764]

It is euill wakyng of a slepyng dog.
[1546 J. Heywood Dialogue of Proverbs i. x. D1^V]

It's best To let a sleeping mastiff rest.
[1681 S. Colvil Whigs' Supplication ii. 27]

Take my advice, and speer [ask] as little about him as he does about you. Best to let sleeping dogs lie.
[1824 Scott Redgauntlet I. xi.]

He would be better off sticking to indifference and undisclosed affection. ‘Let sleeping dogs lie,’ he muttered.
[1976 T. Sharpe Wilt xx.]

‘They don't have to prove it! He's dead. It can't do him any harm now.’ She said distinctly, ‘Let sleeping dogs lie, then.’
[1996 M. Macdonald Death's Autograph ix. 98]

Related to: action and inaction; busybodies

Bibliography of major proverb collections and works cited from modern editions is available here.

Allow inactive problems to remain so, as in Jane knew she should report the accident but decided to let sleeping dogs lie. This injunction to avoid stirring up trouble was already a proverb in the 13th century. It alludes to waking up a fierce watchdog and has been stated in English since the late 1300s.