Musnad Ahmad 2:157

'Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri, Malik ibn Aws ibn al-Hadathan informed me, he said: I exchanged silver for gold with Talhah ibn 'Ubayd Allah. He said: "Wait for me until our treasurer comes to us from al-Ghabah." 'Umar ibn al-Khattab heard this and said: "No, by Allah, you shall not leave him until you take from him his exchange, for I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Gold for silver is usury (riba), except for hand-to-hand (spot exchange).'"