Let sleeping dogs lie: Prov. Do not instigate trouble.; Leave something alone if it might cause trouble. To not talk about things which have caused problems in the past, or to not try to change a situation because you might cause problems.
A: Detective, I believe my husband is involved with another woman.
B: Mrs. Murray, do you love your husband?
A: Yes, I do.
B: Then, let sleeping dogs lie.
A: But I have to know!