I think it is Love in Vein, part one, by Bittenandstaked.
Set pre-Harsh Light of the Day, beginning of Season 4. Spike rescued Buffy, when she was attacked by a demon. She accidentaly claimed him, but they didn't realized it at first. In fact, Angel rescued him after Spike was attacked, but when Buffy found beaten Spike chained in Angel's basement in LA, she considered Angel the attacker... (chapter 8).
http://web.archive.org/web/20070812060902/http://www.talesoftheslayer.com/loveinvein/ loveinvein01.php