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 ourProphet to us and our advice to you.' "
Muwatta Malik Book 31, Hadith 31