Musnad Ahmad 27:1977

Muhammad ibn Ubayd narrated to us, Muhammad—meaning Ibn Ishaq—narrated to us, from Nafi, from Ibn Umar, who said: "The Messenger of Allah, may Allah bless him and grant him peace, forbade the sale of *gharar* (uncertainty/risk)." And that is because the people of the Jahiliyyah (pre-Islamic era) used to trade by the *sharif* (the old she-camel) for the *habal al-habala* (the fetus of the fetus). "So the Messenger of Allah, may Allah bless him and grant him peace, forbade that."