Maliks Muwatta Book 31, Hadith Number 29.
Section : Selling Gold for Silver, Minted and Un-minted.
Yahya related to me from Malik from Musa ibn Abi Tamim from Abu’l Hubab Said ibn Yasar from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, “A dinar for a dinar, a dirham for a dirham, no excess between the two.”