Al Waqi'ah, 76 Ayah

56:76

وَإِنَّهُۥ لَقَسَمٞ لَّوۡ تَعۡلَمُونَ عَظِيمٌ ٧٦

Wa 'Innahu Laqasamun Law Ta`lamūna `Ažīmun

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