Por la boca murió el pez

[ROBINDESBOIS]

This means that sometimes it´s better to keep your mouth shut than say something. You give yourself away.

[poli]

Mum's the word.

ps
A World War 11 expression: loose lips sink ships.

[pjt33]

Better to be silent and thought a fool than to open your mouth and remove all doubt?

[AngelicaDeAlquezar]

@pjt: it can be applied like that, but it's also to avoid talking too much when you have something to hide. :)