Abdur Rahman bin Abi Bakra عبد الرحمن بن أبي بكرة · The Book of Financial Transactions

"Umar said: Dinar for Dinar, Dirham for Dirham, no difference between them, this is the obligation that our Prophet enjoined upon us."

"The Messenger of Allah said: 'Gold for gold, weight for weight, like for like; and silver for silver, weight for weight, like for like. Whoever gives more or takes more has engaged in Riba."'