Abu Hurairah reported the Messenger of Allah ﷺ as saying:
when a fly alights in anyone’s vessel, he should plunge it all in, for in one of its wings there is a disease, and in the other is a cure. It prevents the wing of it is which there is a cure, so plunge it all in (the vessel).

حَدَّثَنَا أَحْمَدُ بْنُ حَنْبَلٍ، حَدَّثَنَا بِشْرٌ، - يَعْنِي ابْنَ الْمُفَضَّلِ - عَنِ ابْنِ عَجْلاَنَ، عَنْ سَعِيدٍ الْمَقْبُرِيِّ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ ﷺ " إِذَا وَقَعَ الذُّبَابُ فِي إِنَاءِ أَحَدِكُمْ فَامْقُلُوهُ فَإِنَّ فِي أَحَدِ جَنَاحَيْهِ دَاءً وَفِي الآخَرِ شِفَاءً وَإِنَّهُ يَتَّقِي بِجَنَاحِهِ الَّذِي فِيهِ الدَّاءُ فَلْيَغْمِسْهُ كُلَّهُ " .