Sahl bin Sa'd bin Malik · The Chapters on Business Transactions

"We used to buy food from troops of riders (i.e., the caravans) without knowing the amount, but the Messenger of Allah (ﷺ) forbade us to sell it until it had been delivered to us."

"I heard 'Umar bin Al-Khattab say: 'The Messenger of Allah (ﷺ) said: "Exchanging gold for gold is usury unless it is done on the spot. (Exchanging) wheat for wheat is usury, unless it is done on the spot. (Exchanging) barley for barley is usury unless it is done on the spot. (Exchanging) dates for dates is usury, unless it is done on the spot."