1
general::
Also, still as a mouse. Silent, without noise, as in She sneaked into the house, quiet as a mouse, or When he heard the news he was still as a mouse. The first of these similes dates from the mid-1500s, the second from the 1300s.
American Heritage Idioms
2
general::
Phrase(s): *quiet as a (church) mouse [and] *quiet as the grave
very quiet. (*Also: as ~.) • You’d better be as quiet as a mouse while Grandma takes her nap so you won’t wake her up. • This town is quiet as the grave now that the factories have closed.
McGrawhill's American Idioms And Phrasal Verbs