Musnad Ahmad 30:2510

Yahya narrated to us, from Ash'ath, from Muhammad, from Abu Salih Dhakwan, from Abu Hurairah, and Abu Sa'id, and Jabir—two of these three—that the Prophet, peace and blessings of Allah be upon him, forbade *al-sarf* (the exchange of gold for silver or currency for currency with a delay).