user mapping hide password
Create a .pgpass file on the postgres server in the $HOME directory of the postgres Linux user. You can then store the password of that database user in that file.
Then you can remove the password option from the user mapping, as Postgres (or more precisely: libpq) will pick up the password from the password file.
Don't forget to set password_required to 'false' on the user mapping unless the user mapping is for a superuser (which is an altogether awful thing to do).