Musnad Ahmad 30:454

Abu Kamil narrated to us, Hammad narrated to us, from Thumamah ibn Abdullah ibn Anas, from Abu Hurairah, that the Prophet, peace and blessings of Allah be upon him, said: "If a fly falls into the vessel of one of you, let him submerge it, for in one of its wings is a disease, and in the other is a cure."