Musannaf Ibn Abi Shayba 13:2530

Abu Bakr narrated to us, he said: Ya'la ibn 'Ubayd narrated to us, from Fudayl ibn Ghazwan, from Ibn Abi Nu'm, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him), who said: "Silver for silver, weight for weight, like for like; and gold for gold, weight for weight. Whatever is in excess is usury (riba). And fruit shall not be sold until its ripeness becomes apparent."