1
general::
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.
American Heritage Idioms
2
general::
Phrase(s): Let sleeping dogs lie.
Prov. Do not instigate trouble.; Leave something alone if it might cause trouble. • Jill: Should I ask the boss if he’s upset at my coming in late in the mornings? Jane: If he hasn’t said anything about it, just let sleeping dogs lie. • I thought I would ask Jill if she wanted me to pay her back right away, but then I decided to let sleeping dogs lie.
McGrawhill's American Idioms And Phrasal Verbs