Musnad Ahmad 2:81

Sufyan narrated to us, from al-Zuhri, that he heard Malik ibn Aws ibn al-Hadthan hear Umar ibn al-Khattab say: The Messenger of Allah (ﷺ) said—and Sufyan said once: he heard the Messenger of Allah (ﷺ) say—: "Gold for silver is usury (riba), except if it is hand-to-hand (ha'an wa ha'an). Wheat for wheat is usury, except if it is hand-to-hand. Barley for barley is usury, except if it is hand-to-hand. Dates for dates is usury, except if it is hand-to-hand."