I'm looking for a fic where Dean finds a HUGE dildo in Sam's bag. It was possibly a Bad Dragon toy. At the future date, he goes looking for it again, but it's not in Sam's bag. At one point he sees a bulge in Sam's stomach and realizes that Sam's had it inside of him all day. Dean tries to keep cool about it but fails. I may be remembering it wrong, but I think Sam fucks Dean with it at the end.

Not sure if this was on AO3 or LJ, but I'm fairly certain it's rather old.