quiet as a mouse - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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.