Musnad Ahmad 30:1536

Aswad ibn Amir narrated to us, Hammad—meaning Ibn Salamah—narrated to us, from Thumamah, 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." Hammad and Habib ibn al-Shahid said: From Muhammad ibn Sirin, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, the like of it.