Mujahid (may Allah have mercy on him) said: I was with Sayyiduna Abdullah bin Umar (may Allah be pleased with them both). 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 I take the extra in return for my labour. Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) forbade him. The goldsmith kept repeating his question and Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) kept forbidding him, until he came to the door of the mosque and mounted his riding animal. Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: "A dinar for a dinar and a dirham for a dirham; there is no excess (tafadul) between them (no increase or decrease). This is the covenant the Prophet (peace and blessings be upon him) took from us, and this is our covenant to you."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10491