Musnad Ahmad 30:440

Ya'la narrated to us, Fudayl—meaning Ibn Ghazwan—narrated to us, from Ibn Abi Nu'm, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "Silver for silver, like for like, weight for weight; and gold for gold, weight for weight, like for like. So whoever gives more, it is usury (riba)."