Musnad Abu Ya'la al-Mawsili 155:329

Abu Bakr ibn Abi Shaybah narrated to us, Ibn Fudayl narrated to us, from his father, from Abu Hazim, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Wheat for wheat, barley for barley, dates for dates, and salt for salt, hand to hand, weight for weight. Whoever gives more or takes more has committed usury (riba), except for what differs in its types."