If you want to redirect both stdout and sterr:
echo "..." | passwd &> /dev/null
which is the equivalent of
echo "..." | passwd > /dev/null 2>&1
which means "redirect stdout to /dev/null and then redirect (duplicate) stderr to stdout". This way you redirect both stdout and stderr to null ... but it might not be enough (it will be in this case I believe). But theoretically the program might write directly to terminal. For example this script
$ cat test.sh
echo stdout
echo stderr 1 1>&2
echo stderr 2 >/dev/stderr
echo stderr 3 >/dev/fd/2
echo bad luck > /dev/tty
$ ./test.sh &> /dev/null
bad luck
To get rid even of this output you must force the program to run in pseudo terminal, for example http://empty.sourceforge.net/ . But that is just a side note &> /dev/null will work fine.