Musnad Abu Ya'la al-Mawsili 2:70

Daud ibn Rashid narrated to us, Abbad ibn al-Awwam narrated to us, Sufyan ibn Husayn narrated to us, from al-Zuhri, from Malik ibn Aws ibn al-Hadathan, that Umar ibn al-Khattab sold one hundred dinars for silver (dirhams) to Talhah ibn Ubayd Allah. Umar said, "Hand it over to me." Talhah said, "Wait until my agent comes from al-Ghabah." Umar said, "No, by Allah, I will not let you go until you hand it over to me, for I heard the Messenger of Allah (ﷺ) say: 'Gold for silver is usury (riba), except if it is hand-to-hand (spot exchange).'"