It was narrated that Zuhri heard Malik bin Aws bin Hadathan say:
"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). (RU)
English reference (RU): Vol. 3, Book 12, Hadith 2259 (RU)
Arabic reference (RU): Book 12, Hadith 2344 (RU)