I think it was the body of Horus they destroyed, not the clone, which wasn't completed. Abaddon is said to be a clone, but that's a bit of a sticky point, which GW are neither here nor there about, and just throw in to up the word count in articles.
The most I've read is in the Index Astartes article. Maybe it's in with the old 2nd edition stuff, the oldest WD's I have only go back to where Abaddon was introduced