Nahj al-Balāgha

Hadith 2066
repentance
٤٧٧. وَقَالَ (عليه ‏السلام): أَشَدُّ الذُّنُوبِ مَا اسْتَخَفَّ بِهِ صَاحِبُهُ.
477. Amīr al-mu’minīn, peace be upon him, said: The worst sin is that which the committer takes lightly.
Source: al-Sharīf al-Raḍī