“But if they turn away, then say, 'I have warned you of a thunderbolt like the thunderbolt [that struck] Ad and Thamud.'”
This is the exact ayah, per the well-documented narration (and this event's own title), that the Prophet ﷺ recited to Utbah ibn Rabi'ah -- who turned pale at the mention of a "thunderbolt like that of Ad and Thamud."