message-yank-original
message-yank-original is an interactive autoloaded compiled Lisp function in `message.el
'.
(message-yank-original &optional ARG)
Insert the message being replied to, if any.
Puts point before the text and mark after.
Normally indents each nonblank line ARG spaces (default 3). However,
if `message-yank-prefix' is non-nil, insert that prefix on each line.
This function uses `message-cite-function' to do the actual citing.
Just C-u as argument means don't indent, insert no
prefix, and don't delete any headers.