knock on wood
knock on wood {v. phr.}
To knock on something made of wood to keep from having bad luck. — Many people believe that you will have bad luck if you talk about good luck or brag about something, unless you knock on wood; often used in a joking way.
Charles said, "I haven't been sick all winter." Grandfather said, "You'd better knock on wood when you say that."
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Source: A Dictionary of American Idioms