Hanash al-San'ani (may Allah have mercy on him) narrates from Sayyiduna Ruwayfi' bin Thabit (may Allah be pleased with him) that during a military campaign of the people towards the Maghrib (North Africa) he said: The Messenger of Allah (peace and blessings be upon him) said during the campaign of Khaybar: "It has reached me that you sell a mithqal (of gold) for half or two-thirds, whereas that is not right except a mithqal for a mithqal and weight for weight."
Hadith Referenceالمعجم الكبير للطبراني / باب الراء / 4464