مسند أحمد

Malik bin Aws bin al-Hadathan said: I exchanged some silver for gold with Talhah bin ‘Ubaidullah, and he said: Wait for me until our storekeeper comes from al-Ghabah, `Umar bin al-Khattab heard that and said: No, by Allah! Do not leave him until you take your exchange from him, for I heard the Messenger of Allah (ﷺ) say: `Gold for silver is riba unless it is `here, take this` and `here, take this` [i.e., exchanged on the spot].”