@mart2306Did you see it on SOL or elsewhere. While SOL does have some fanfic, it's never been super popular here. The problem is that the big fanfic sites tend to discourages, or in some cases outright ban, fanfic with explicit content. Moreover, the big sites have a LOT of content, often with limited sorting options, so without a title finding a specific story can take ages.
If it wasn't on SOL and was explicit, try Archive of Our Own. Once there, select TV series, then Buffy, and make heavy use of the filter sidebar. Especially make sure to select English at the bottom of the sidebar. There are over 37k BVS stories on the site.