Mujahid narrates that I was with Sayyiduna Abdullah bin Umar (may Allah be pleased with them). A goldsmith came to him and said, "O Abu Abdur Rahman! I make jewelry, so I deduct my wages from it (thus its weight becomes less due to the deduction), then I sell it for more than its weight." Sayyiduna Abdullah bin Umar (may Allah be pleased with them) forbade him from this action. The goldsmith kept asking the same question repeatedly, and Sayyiduna Abdullah (may Allah be pleased with him) kept forbidding him, until they reached the door of the mosque or to the place to mount the ride. Then Sayyiduna Abdullah (may Allah be pleased with him) said: "In the exchange of dinar for dinar and dirham for dirham, any increase between them is not permissible. This is an emphasis from our Prophet (ﷺ) to us, and we are emphasizing it to you."
Hadith Referenceالسنن المأثورة / باب في البيوع / 213
Hadith Takhrijسنن نسائى، البيوع، باب بيع الدرهم بالدرهم، رقم : 4568 وقال الالبانی: صحيح بما قبله، مؤطا امام مالك ، البيوع، باب بيع الذهب بالفضة تبرا وعينا، رقم : 31 ۔