Musannaf Ibn Abi Shayba 13:550

Abu Bakr narrated to us, he said: Ibn Idris narrated to us, from Ubayd Allah ibn Umar, from Abu al-Zinad, from al-A'raj, from Abu Hurayrah, that the Prophet, may Allah bless him and grant him peace, forbade the sale of *gharar* (uncertainty/risk).