Musannaf Ibn Abi Shayba 35:455

Sufyan ibn 'Uyaynah narrated to us, from al-Zuhri, that he heard Malik ibn Aws ibn al-Hadathan saying: I heard Umar saying: The Messenger of Allah (ﷺ) said: "Wheat for wheat is usury (riba), except if it is hand-to-hand (ha'an wa ha'). And barley for barley is usury (riba), except if it is hand-to-hand (ha'an wa ha')."