Sahih Muslim
The Book of Transactions (Kitab Al-Buyu)
Book Number: 010 / Hadith Number: 3866
Fadala b. 'Ubaid reported: We were in the company of Allah's Messenger ( may peace be upon him) on the day (of the Victory of) Khaibar, and made transaction with the Jews for the 'uqiya of gold for the dinars or three (gold coins), whereupon Allah's Messenger (may peace be upon him) said: Do not sell gold for gold but for equal weight