Raise a hand straight above your head to indicate that you expect to stop. This indication supersedes the indication and call to slow.
Optional call: “Stopping!”
While optional, the call of “Stopping!” can be absolutely necessary if the nature of the stop is sudden or sharp, and if you therefore haven’t got the time to make the signal. This can make the difference between a safe stop and a potentially very dangerous situation, so ensure the call is loud, sharp and urgently made with as much notice as possible.