A: Has she found out where the money went?

B: No. She decided to let sleeping dogs lie and not to ask her son about the missing money.

Note: This idiom means to “let a matter or person which at the present is at rest stay at rest, rather than to create a disturbance by bringing the matter up again.” Chaucer wrote this in just the reverse form: It is nought good a slepyng hound to wake. (“Troylus and Crisedye,” 1374). It was recorded in the same form some 200 years later by John Heywood (“A Dialogue Conteynyng Prouerbes and Epigrammes,” 1592): It is ill wakyng of a sleapyng dogge. But by the time of Charles Dickens (“David Copperfield,” 1850) it had been turned into its modern form.