Musnad Ahmad 31:64

Yahya ibn Sa'id narrated to us, from Ash'ath, from Muhammad, from Abu Salih Dhakwan, from Abu Hurayrah, and Abu Sa'id, and Jabir—two of these three: "That the Prophet, may Allah bless him and grant him peace, forbade *al-sarf* (the exchange of gold for silver or currency for currency with a delay in delivery)."