It is narrated from Wardan the Roman that he asked Sayyiduna Ibn Umar (may Allah be pleased with them both): "I am a goldsmith. I make jewelry and sell it. I deduct my wage from the gold itself" (i.e., whatever the original weight of the gold was, I sell it for the same amount, I do not take a separate wage; for example, if I made jewelry from one dinar of gold, I deducted my wage from that gold and then sold the dinar further). So Sayyiduna Ibn Umar (may Allah be pleased with them both) said: "If the transaction of gold (jewelry) is with gold (dinar), then any increase or decrease is not permissible."
Hadith Referenceالسنن المأثورة / باب في البيوع / 214
Hadith Takhrijالسنن الكبرى للبيهقي : 275/5 ، التمهيد لابن عبدالبر : 247/2، معانی الآثار للطحاوي : 66/4، حدیث صحیح ۔