Musnad Ahmad 30:1365

Yunus narrated to us, Laith narrated to us, from Muhammad, from al-Qa'qa', from Abu Salih, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, that he said: "Indeed, in one of the wings of a fly is a disease, and in the other is a cure. So if it falls into the vessel of one of you, it protects itself with that which contains the disease, so let him submerge it, then remove it."