Fadala bin 'Ubaid bin Naqid · The Book of Financial Transactions

"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…

"The messenger of Allah said: 'Gold for gold, of equal measure; silver for silver, of equal measure; salt for salt, dates for dates wheat for wheat, barley for barley, like for like. Whoever gives more or takes more has engaged in Riba."' (Sahih) The wording is that of Muhammad Yaqub did not mention - "Wheat for wheat".