Classification: Sahih (authentic)
Narrated Abu Hurairah Allah be pleased with him that the Prophet Muhamad (peace and blessings upon him) said: "If a housefly falls into the drink of anyone of you, he should immerse it completely in the liquid, then remove it, for one of its wings has the disease and the other has the cure." (Sahih Bukhari, 3320)
In another narration of this hadith related by Abu Dawood, it also mentions that the fly protects itself with the wing that has the disease under it.
This means that when the fly falls into a container of food or drink, it falls with its wing that has the disease into the food/drink first, while the other wing with the cure might remain outside the drink. Therefore, we should immerse the fly completely into the food or drink before removing it.