Musannaf Ibn Abi Shayba 13:2541

Abu Bakr narrated to us, he said: Ibn Ishaq narrated to us, from Wuhayb, from Yahya ibn Abi Ishaq, from Abd al-Rahman ibn Abi Bakra, from his father, who said: The Messenger of Allah (may Allah bless him and grant him peace) forbade us from selling gold for gold and silver for silver, except equal for equal. And he commanded us to sell gold for silver and silver for gold however we wished.