keep quiet
stay quiet
stay silent

Once again - if you have nothing more than stupidity to offer, save your own face by at least staying silent.

After staying silent a moment or two he started off again.

Few, if any, have rushed to defend him, staying silent at best.

She would have stayed silent, but the pressing air did not allow it.

She had stayed silent during the Simpson controversy, never speaking to the press.

She didn't know what to say and stayed silent, her face hot.

I knew where this was going, but stayed silent.

She never knew how to respond to that sort of talk so stayed silent.

Jackie stayed silent, but hoped what Lily said was true.

And if Nightingale were here, we would be singing instead of staying silent.