Musnad Ahmad 33:61

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 Hurairah, that they: "Forbade *al-sarf* (the exchange of gold for gold or silver for silver with a delay in delivery)." And two men among them attributed it to the Prophet (raised it to him).