Surah Al-Waqi'ah, ayah 76

وَإِنَّهُۥ لَقَسَمٞ لَّوۡ تَعۡلَمُونَ عَظِيمٌ ٧٦

Wa innahū laqasamul law ta'lamūna 'azīm

And indeed, it is an oath - if you could know - [most] great.