Sahih Muslim Book 10, Hadith Number 3867.

Hanash reported: We were along with Fadala b. Ubaid (Allah be pleased with him) in an expedition. There fell to me and my friend’s lot a necklace made of gold, silver and jewels. I decided to buy that. I asked Fadala b. ‘Ubaid, whereupon he said: Separate its gold and place it in one pan (of the balance) and place your gold in the other pan, and do not receive but equal for equal, for I heard Allah’s Messenger (may peace be upon him) as saying: He who believes in Allah and the Hereafter should not take but equal for equal.