Musnad Abu Ya'la al-Mawsili 2:10

Abu Khaythama, Ishaq ibn Isma'il al-Taliqani, and al-Qawariri narrated to us, they said: Sufyan narrated to us, from al-Zuhri, from Malik ibn Aws ibn al-Hadathan, that he heard Umar ibn al-Khattab saying: The Messenger of Allah (may Allah bless him and grant him peace) said: "Gold for silver is usury (riba), except for 'hand to hand' (haa' wa haa'). Barley for barley is usury, except for 'hand to hand'." And in the hadith of Ishaq: "And dates for dates is usury, except for 'hand to hand'." And in it, from Malik ibn Aws: "I heard Umar informing that the Prophet (may Allah bless him and grant him peace)..."