Musnad Ahmad 31:62

Muhammad ibn Ja‘far narrated to us, Sa‘id narrated to us, that Muhammad narrated that Dhakwan, Abu Salih, narrated from Abu Sa‘id al-Khudri, Jabir ibn ‘Abdullah, and Abu Hurayrah: That they forbade *al-sarf* (the exchange of gold for gold or silver for silver with a difference in weight or delay in delivery). And two men among them attributed it to the Prophet of Allah, may Allah bless him and grant him peace.