Muslims affirm that riba, usury, and in the dominant view any guaranteed increase on a loan of money, is forbidden. The Quran condemns it in escalating terms, contrasting lawful trade with riba and warning of divine war against those who persist (2:275-279), and instructs creditors to remit hardship. The prohibition is treated as one of the gravest of the major sins and underlies the whole edifice of Islamic finance, which seeks profit through shared risk and real assets rather than interest.
Classical jurisprudence built its detailed law of riba on a hadith, transmitted through Ubada ibn al-Samit, listing six commodities, gold, silver, wheat, barley, dates, and salt, that may be exchanged for their own kind only in equal quantity and hand to hand; from this text jurists derived two distinct categories, riba al-fadl, a forbidden excess in a same-kind barter exchange, and riba al-nasiah, the delay-based increase most people mean by interest on a loan. The schools differed over how far to extend the six-commodity ruling by analogy to other goods, especially modern currency, a question that remains central to how contemporary Islamic finance decides which transactions are permissible.