Sayyiduna Fadalah bin Ubayd (may Allah be pleased with him) narrated: On the day of Khaybar I bought a necklace of gold and beads for twelve dinars. Then I separated it, and the gold in it came to more than twelve dinars. I mentioned this to the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) said: "Gold may not be sold for gold until it is separated."
Hadith Referenceالمعجم الكبير للطبراني / باب الفاء / 15865