Karayb bin Abi Muslim كريب بن أبي مسلم الهاشمي · The Chapters on Business Transactions

"I came saying. 'Who will exchange Dirham?' Talhah bin 'Ubaidullah, who was with 'Umar bin Khattab, said: 'Show us your gold, then come to us; when our treasure comes, we will give you your silver.' 'Umar said: 'No, by Allah, you will give him silver (now), or give him back his gold, for the Messenger of Allah (ﷺ) said: "Silver for gold is usury, unless it is exchanged on the spot."'