Nahj al-Balāgha
Hadith 101وَإِنَّ القُرآنَ ظَاهِرُهُ أَنِيقٌ وَبَاطِنُهُ عَمِيقٌ، لاَ تَفْنَى عَجَائِبُهُ، وَلاَتَنْقَضِي غَرَائِبُهُ، وَلاَ تُكْشَفُ الظُّلُمَاتُ إلاَّ بِهِ.
Certainly the outside of the Qur’ān is wonderful and its inside is deep (in meaning). Its wonders will never disappear, its amazements will never pass away and its intricacies cannot be cleared except through itself.