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