Maliks Muwatta Book 31, Hadith Number 36.

Section : Selling Gold for Silver, Minted and Un-minted.

Yahya related to me from Malik that he had heard that al-Oasim ibn Muhammad said, ”Umar ibn al-Khattab said, ‘A dinar for a dinar, and a dirham for adirham, and a sa for a sa. Something to be collected later is not to be sold for something at hand.'”