gnus-cite-attribution-prefix

gnus-cite-attribution-prefix is a variable defined in `gnus-cite.el'.
Its value is

"In article\\|in <\\|On \\(Mon\\|Tue\\|Wed\\|Thu\\|Fri\\|Sat\\|Sun\\),\\|----- ?Original Message ?-----"

Documentation:
*Regexp matching the beginning of an attribution line.

You can customize this variable.