The Peak of Eloquence · Hadith 2251 (#Passage #610)

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.

Gradings are classical jarḥ-wa-taʿdīl, name-matched to a rijāl index (not individually re-verified).