﴿ فَإِنْ أَعْرَضُوا۟ فَقُلْ أَنذَرْتُكُمْ صَٰعِقَةًۭ مِّثْلَ صَٰعِقَةِ عَادٍۢ وَثَمُودَ ﴾

“BUT IF they turn away, say: “I warn you of [the coming of] a thunderbolt of punishment like the thunderbolt [that fell upon the tribes] of Ad and Thamud!””

Please check ayah 41:18 for complete tafsir.