Ignorance of religious law leads to usury
Nahj al-Balāgha|Volume 1|Book 3|Chapter 447
٤٤٧. وَقَالَ (عليه السلام): مَنِ اتَّجَرَ بِغَيْرِ فِقْهٍ فَقَدِ ارْتَطَمَ فِي الرِّبَا.
447. Amīr al-mu’minīn, peace be upon him, said: Whoever trades without knowing the rules of religious law will be involved in usury.