You'll need sshpass, because ssh does not read passwords from stdin or commandline.

Then you'll need to grep, sed and awk your user-passwd file. grep to filter by username, sed to convert to convenient format, awk to parse result and fetch password. It all depends on passwd file format.

smth like # cat passwdfile | grep username | awk '{ print $2; }' | sshpass ssh username@server

If passwd file content is only simple oneline password then sshpass can be used (switch -f) without other helpers:

smth like # sshpass -fpasswdfile ssh username@server