I usually don't say anything if I can give the person enough room that they can't hit me as I'm passing. That's my usual mode of passing, lots of distance. I usually do something to make sure they hear me coming. If I can't do that, I say "passing" far enough back that they can recover from being startled before I get there. I have had people sneak up right behind me and say, "on your left" and it almost never goes well. I have bells on a couple of my bikes, but I never use them