Help needed in removing warning: "Untrusted packages could ..."

["Amogh Hooshdar" <amoghhooshdar@gmail.com>]

Whenever I try to install something using aptitude install, I get this warning:-

-------
WARNING: untrusted versions of the following packages will be installed!

Untrusted packages could compromise your system's security.
You should only proceed with the installation if you are certain that
this is what you want to do.

  locate

Do you want to ignore this warning and proceed anyway?
-------

Is there anything I can do to suppress this warning like making the
system believe that I trust ftp.us.debian.org or whatever mirror I am
using.