I thought, ok, let sleeping dogs lie! then I wondered what google would say about that phrase I have heard all my life.
"Let sleeping dogs lie -- 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 or arousing the person. (in this case person = dogs)
'It is nought good a slepyng hound to wake.' ('Troylus and Crisedye,' 1374)
- and it was still so recorded some two hundred years later by John Heywood ('A Dialogue Conteynyng Prouerbes and Epigrammes,' 1592),
'It is ill wakyng of a sleapyng dogge.' (my personal favorite)