Musnad Ahmad 30:2593

Waki’ narrated to us, from Ibrahim ibn al-Fadl, from Sa’id ibn Abi Sa’id, from Abu Hurayrah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "If a fly falls into the food or drink of one of you, let him immerse it, then remove it; for in one of its wings is a disease, and in the other is a cure, and it presents the disease first."