Musnad Ahmad 30:3149

'Abd al-Rahman narrated to us, Sufyan narrated to us, from Sa'd ibn Ibrahim, from 'Umar ibn Abi Salamah, from his father, from Abu Hurayrah, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade al-muhaqalah and al-muzabanah." And al-muhaqalah is [the exchange of] wheat for wheat.