quiet as a mouse
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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.