Musannaf Ibn Abi Shayba 13:2543

Abu Bakr narrated to us, he said: Affan narrated to us, he said: Shu'bah narrated to us, he said: Habib ibn Abi Thabit informed us, he said: I heard Abu al-Minhal say: I asked al-Bara' ibn 'Azib and Zayd ibn Arqam about currency exchange (al-sarf), and both of them said: The Messenger of Allah (may Allah bless him and grant him peace) forbade the selling of silver for gold on credit.