"O people, you have invented kinds of transactions, I do not know what they are, but make sure it is gold for gold, of the same weight, or silver for silver, of the same weight. There is nothing wrong with selling silver for gold, hand to hand, giving more silver than gold, but no credit is allowed. When you sell wheat for wheat and barley for barley, it should be measure for measure, but there is nothing wrong with selling barley for wheat, hand to hand, giving more barley than wheat, but no credit is allowed. And when you sell dates for dates, it should be measure for measure" And he mentioned salt, "measure for measure, and whoever gives more or asks for more has engaged in Riba.
The Book of Financial Transactions - Sunan an-Nasa'i 4563
This narration from the Prophet Muhammad (peace be upon him) establishes the fundamental principles governing riba (usury/interest) in Islamic commercial law, particularly addressing the prohibition of riba al-fadl (excess in exchange) and riba al-nasi'ah (delay in exchange).
Commentary on Monetary Exchange (Sarf)
The Prophet's instruction "gold for gold, of the same weight, or silver for silver, of the same weight" establishes that when exchanging identical monetary commodities (thaman), they must be equal in quantity and exchanged hand-to-hand (taqabud).
The permission to exchange silver for gold with disparity in quantity, provided it is hand-to-hand, recognizes these as different monetary species while maintaining the prohibition of deferred payment, which constitutes riba al-nasi'ah.
Commentary on Commodity Exchange (Bay')
The ruling for wheat, barley, dates, and salt follows the same principle: when exchanging identical foodstuffs measured by volume (kayl), they must be equal in measure and exchanged immediately.
The permission to exchange barley for wheat with disparity recognizes them as different species while prohibiting credit sales, which would constitute riba.
Legal Wisdom and Application
These six commodities (gold, silver, wheat, barley, dates, salt) became the foundation for the classical Islamic theory of riba, with scholars extending the ruling to all items sharing their underlying causes ('illah): being monetary standards or staple foods measured by weight or volume.
The final warning "whoever gives more or asks for more has engaged in Riba" emphasizes the gravity of violating these principles, as such transactions exploit need and create injustice in society.