Click the ellipsis (three dots) link at the top of the message (next to the Forward link).
Select View Message Details from the dropdown menu. This will bring up a window containing the full header information.
Click in the window and select all the text, then copy it to your clipboard (Ctrl-C in Windows or Cmd-C in Mac OS). Click on the Forward link, then paste (Ctrl-V in Windows or Cmd-V in Mac OS) the header information at the top of the forwarded email.