What is the ruling when a husband says to his wife, who is currently pure (tahir), "If you menstruate, you are divorced"?

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The divorce takes effect immediately if the woman observes blood during a period where menstruation is typically possible, and this observation is treated as menstruation for rulings like prohibiting prayer. If it later becomes apparent that the bleeding was not menstruation because it lasted less than the minimum duration for a period, the divorce is ruled as not having occurred. This view is held by Al-Thawri, Al-Shafi'i, and the companions of Al-Ra'y (Ahl al-Ra'y).