Musnad Abu Ya'la al-Mawsili 112:11

Muhammad ibn Abdullah ibn Numayr narrated to us, my father narrated to us, from Ismail ibn Muslim, from al-Hasan and Qatadah, from Anas, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not trade in *gharar* (uncertainty/risk)."