Muhammad bin Ajlan al-Madni محمد بن عجلان المدني · The Chapters on Business Transactions

"I heard 'Umar say: "The Messenger of Allah (ﷺ), said: 'Gold for silver is usury, unless it is exchanged on the spot."' (Sahih) Abu Bakr bin Abu-Shaibah said: "I heard sufyan saying: 'Gold for silver." memorize (this).