Не буди лихо, пока оно тихо

(ne budí líkho, poká onó tíkho).

Literally:

don’t wake Likho* up while it’s quiet.

Likho is an embodiment of evil Dólya (Доля) - Nedólya - in eastern Slavic mythology. Dolya is an embodiment of happiness and good luck. Thus, Likho represents misfortune and it is depicted as a one-eyed creature: a giant (cf.: Cyclops) or an old woman.

English equivalents:

• don’t trouble trouble until trouble troubles you;

• let sleeping dogs lie.