Sahih Muslim Book 10, Hadith Number 3864.

Chapter : The selling of the camel and stipulation of riding on it.

Fadila b. ‘Ubaid (Allah be pleased with him) reported: I bought on the day (of the Victory of Khaibar) a necklace for twelve dinars (gold coins). It was made of gold studded with gems. I separated (gold from gems) in it, and found (gold) of more (worth) than twelve dinars. I made a mention of it to Allah’s Apostle (may peace be upon him), whereupon he said: It should not be sold unless it is separated.