Musnad Ahmad 30:1763

Muhammad ibn Ubayd narrated to us, Ubayd Allah narrated to us, from Abu al-Zinad, from Abd al-Rahman al-A'raj, from Abu Hurayrah: "That the Messenger of Allah (peace and blessings of Allah be upon him) forbade the sale of *gharar* (uncertainty/risk) and the sale of *al-hasah* (pebble sale)."