1. Islam prohibits Riba (interest/usury) based on several Quranic verses that warn of severe punishment for those who consume or deal in Riba.
2. The Quran clarifies that trade is permitted but Riba is forbidden, and distinguishes between the two.
3. Hadith further emphasize the prohibition and consider even small amounts of knowingly consumed Riba to be graver than major sins in God's sight.