initial-scratch-message

initial-scratch-message is a variable defined in `startup.el'.
Its value is

";; This buffer is for notes you don't want to save, and for Lisp evaluation.\n;; If you want to create a file, visit that file with C-x C-f,\n;; then enter the text in that file's own buffer.\n\n"

Documentation:
Initial message displayed in *scratch* buffer at startup.
If this is nil, no message will be displayed.

You can customize this variable.