The narration "If a fly falls in the drink of any of you, let him dip it in"
30-8-2010 | IslamWeb
Question:
How authentic is the Hadeeth about the fly?
Answer:
All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Him, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Abu Daawood and Ibn Hibbaan 
narrated on the authority of Abu Hurayrah 
that the Prophet, sallallaahu ‘alayhi wa sallam, instructed that if a fly fell into someone’s drink, he should immerse it completely in the liquid and then remove it; this is because one of its wings contains a disease and the other, its antidote. The first wing to fall in is that which has a bug, but if one dips the fly and then removes it, the drink will return to its original state. This is also related by Abu Sa‘eed and recorded by Ahmad and An-Nasaa’i . However, the narrations of Al-Bukhari and Al-Bayhaqi do not state that the wing which has disease falls in first.
The fact, though, that a fly bears a disease on one wing and its cure in the other is an established one and transmitted from the Prophet, sallallaahu ‘alayhi wa sallam. Anyone who rejects his reputable Hadeeths, knowing that they are recognized [to be accurate], is considered to have gone out of the fold of Islam.
If he refuses to accept them because of their contradiction with science or his mind does not believe them, then he is in great danger. Scientific rules are experimental [in nature] and are not fixed. Something may be proven today and then negated tomorrow, or vice versa.
Furthermore, there are many unanswered questions that scientists have failed to answer. For instance, they do not, and will not, know anything about the soul, whose separation from the body causes it to be a motionless corpse. Allaah the Almighty rightly Says (what means), {And mankind have not been given of knowledge except a little.} [Quran 17:85]
Allaah Knows best.