don't tell a soul
Don't reveal this information to anyone else. Greg got the promotion, but don't tell a soul—the boss will officially announce it later today.
not tell a soul
To not reveal some confidential information to a single other person. Often spoken as a command or a promise. I heard Greg is getting fired, but don't tell a soul—I don't think even he knows yet. I'm so sorry, I didn't mean to pry! I won't tell a soul!
Don't tell a soul.
Please do not tell anyone this gossip. Bill: Is your brother getting married? Sally: Yes, but don't tell a soul. It's a secret. Mary: Can you keep a secret? John: Sure. Mary: Don't tell a soul, but Tom is in jail.