Sayyiduna Abu Rafi' (may Allah be pleased with him) narrates: I used to do goldsmith's work for the wives of the Prophet (peace and blessings be upon him), and they told me that they heard the Prophet (peace and blessings be upon him) say: "Gold for gold and silver for silver are to be exchanged weight for weight. Whoever gives extra or seeks extra has engaged in usury (riba)."
Ahmad recorded it; its chain contains a narrator, Yahya al-Bakka', who is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب البيوع / 6564