Wiktionary
quiet as a mouse
a. (context simile English) Very quiet, so as to not be heard
Usage examples of "quiet as a mouse".
Nights when I couldn't sleep because of heat I used to sneak quiet as a mouse and bare as a frog out the back door, being oh so careful not to let my parents hear, and walk naked in the dark, with grass cool on my feet and the soft night breeze velvet on my skin.
Watching her, chuckling at the process and also helplessly at what had occurred to him that afternoon (and which he intended to repeat, creeping quiet as a mouse when all were asleep), George didn't notice Cloud fall silent before the completing pattern.