Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Gold for gold, equal in weight, like for like, and silver for silver, equal in weight. Whoever gives more or asks for more has taken usury (riba)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10547
Mujahid (may Allah have mercy on him) said: I was going around with Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) when a goldsmith came and said: O Abu Abd al-Rahman! I fashion gold into jewellery, then I sell some of it for more than its weight, and so I take my wage for my work in it. Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) forbade him from that. The goldsmith kept repeating his question and Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) kept forbidding him, until he reached the door of the mosque or his mount. Then Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: "Dinar for dinar, and dirham for dirham; there is no excess between them." This was the covenant of our Prophet (peace and blessings be upon him) to us, and this is our covenant to you.
(b) The hadith of Sayyiduna Mu'awiyah (may Allah be pleased with him) has already passed, when he sold a drinking vessel of gold or silver for more than its weight and Sayyiduna Abu al-Darda' (may Allah be pleased with him) forbade him; and the prohibition of this is also reported from Sayyiduna Umar bin al-Khattab (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10548
Abu Rafi' (may Allah be pleased with him) said: Umar bin al-Khattab (may Allah be pleased with him) used to sit with me and teach me a verse. I would forget it, and I would call out to him: O Commander of the Believers! I have forgotten it. He would come back and teach it to me. I said to him: I fashion gold into jewellery and I sell it at its weight. Should I take my wage?
He said: Sell gold for gold only equal in weight, and silver for silver equal in weight. Do not take anything extra.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10549