Sahih Muslim Book 10, Hadith Number 3866.

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

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