Muwatta 1860

Malik says: whoever weighs gold for gold, or silver for silver, and there is a mithqal of excess between them, then gives its silver value to his partner, and the partner refuses to take it: this is ugly and a means to riba. If it is permitted for him to take the mithqal at its value, as if he bought it separately, it would be permitted for him to take the mithqal at its value many times, because permitting such a sale between him and his partner would make it so. Malik said: if he sold that mithqal alone, with nothing else with it, he would not take for it a tenth of the price he took for it, because that would be permitted sale, and that is a means to make lawful what is forbidden. ¹

Belongs to the Muwatta, Book of Sales. Source: the Muwatta. Mentions Malik ibn Anas. Built on in the chapters on weighing exchange (muratala) and riba.