Loose lips sink ships

Loose lips, sink ships. (Don't talk carelessly because you don't know who is listening) ~ Болтун — находка для шпиона. Язык мой — враг мой.

Example: You never know who is going to hear what you say and how they will use what they hear. Remember, loose lips sink ships.

Origin:
From wartime — a World War II warning to keep your mouth shut. The idea is that not being cautious with information in correspondence might allow the enemy to obtain information and undermine the war effort. Literally, "Don't reveal even the location of a loved one on a ship, because the location could be communicated to the enemy by a spy."

[Me and my big mouth!]