Musnad Ahmad 2:232

'Uthman ibn 'Umar and Abu 'Amir narrated to us, they said: Malik narrated to us, from al-Zuhri, from Malik ibn Aws ibn al-Hadathan, who said: I brought some dinars of mine and wanted to exchange them. Talhah ibn 'Ubayd Allah met me, and he exchanged them and took them. He said: "Wait until my treasurer comes" — Abu 'Amir said: "from al-Ghabah," and he said regarding all of them: "hand to hand." I asked 'Umar ibn al-Khattab about that, and he said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: "Gold for silver is usury, except for hand to hand; wheat for wheat is usury, except for hand to hand; barley for barley is usury, except for hand to hand; and dates for dates is usury, except for hand to hand."