Bulugh al-Maram, Book 1, Hadith 16 (English) ⓘ

Narrated Abu Huraira: Narrated Abu Huraira (rad): Allah’s Messenger said: “When a fly falls in the drink of one of you, he should fully dip it and then throw it away because there is disease in one of its wings and cure in the other”. [Reported by Al-Bukhari]

وَعَنْ أَبِي هُرَيْرَةَ ‏- رضى الله عنه ‏- قَالَ: قَالَ رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-{ إِذَا وَقَعَ اَلذُّبَابُ فِي شَرَابِ أَحَدِكُمْ فَلْيَغْمِسْهُ, ثُمَّ لِيَنْزِعْهُ, فَإِنَّ فِي أَحَدِ جَنَاحَيْهِ دَاءً, وَفِي اَلْآخَرِ شِفَاءً } أَخْرَجَهُ اَلْبُخَارِيُّ (21)‏.‏ ‏1 ‏- صحيح.‏ رواه البخاري (3320)‏، (5782)‏.‏

and Abu Da’ud who added: “It (the fly) protects itself with the diseased wing (by dipping it first in a drink).

وَأَبُو دَاوُدَ, وَزَادَ: { وَإِنَّهُ يَتَّقِي بِجَنَاحِهِ اَلَّذِي فِيهِ اَلدَّاءُ } (22)‏.‏ ‏2 ‏- سنن أبي داود (3844)‏ وإسنادها حسن.‏