Nahj al-Balagha Hadith n. 477

Selections from The Sayings and Preaching of Amir al-Muminin Ali ibn Abi Talib (Peace Be Upon Him) Including His Replies to Questions and Maxims Expressed for Various Purposes.
Translated by Sayyid Ali Reza

وَقَالَ (عليه ‏السلام):

Amir al-mu'minin, peace be upon him, said:

أَشَدُّ الذُّنُوبِ مَا اسْتَخَفَّ بِهِ صَاحِبُهُ.

The worst sin is that which the committer takes lightly.