Maliks Muwatta Book 31, Hadith Number 31.
Section : Selling Gold for Silver, Minted and Un-minted.
Yahya related to me from Malik from Humayd ibn Qays al-Makki that Mujahid said, “I was with Abdullah ibn Umar and an artisan came to him and said, ‘Abu Abd ar-Rahman – I fashion gold and then sell what I have made for more than its weight. I take an amount equivalent to the work of my hand.’ Abdullah forbade him to do that, so the artisan repeated the question to him, and Abdullah continued to forbid him until he came to the door of the mosque or to an animal that he intended to mount. Then Abdullah ibn Umar said, ‘A dinar for a dinar, and a dirham for a dirham. There is no increase between them. This is the command of our Prophet to us and our advice to you.'”