Musnad Ahmad 971:1

Abu al-Nadr narrated to us, Abu Ja'far narrated to us, from Yahya al-Bakka', from Abu Rafi', who said: I used to work as a goldsmith for the wives of the Prophet, may God bless him and grant him peace. They told me that they heard the Messenger of God, may God bless him and grant him peace, saying: "Gold for gold, and silver for silver, weight for weight. Whoever gives more or asks for more has engaged in usury (riba)."