Function frame_system::ensure_signed

pub fn ensure_signed<OuterOrigin, AccountId>(
    o: OuterOrigin
) -> Result<AccountId, BadOrigin> where
    OuterOrigin: Into<Result<RawOrigin<AccountId>, OuterOrigin>>, 

Ensure that the origin o represents a signed extrinsic (i.e. transaction). Returns Ok with the account that signed the extrinsic or an Err otherwise.