The Messenger of Allah ﷺ said, "When a fly falls into one of your vessels, dip it in completely, for one of its wings carries disease and the other carries a cure. It shields itself with the wing that has the disease, so submerge it entirely."
حدثنا أحمد بن حنبل حدثنا بشر يعني ابن المفضل عن ابن عجلان عن سعيد المقبري
قال رسول الله ﷺ إذا وقع الذباب في إناء أحدكم فامقلوه فإن في أحد جناحيه داء وفي الآخر شفاء وإنه يتقي بجناحه الذي فيه الداء فليغمسه كله
[Sahih] Ibn Khuzayma related it, no. 105, from the hadith of Bishr bin Mufaddal, through the same route, and it is found in Musnad Ahmad, 2/229–230. It has corroborating narrations in Bukhari, no. 3320, and Tahawi in Mushkil Athar, 4/283, and others besides them.