Musnad Abu Ya'la al-Mawsili 42:13

Zuhayr narrated to us, Yahya bin Sa'id narrated to us, Ibn Abi Dhi'b informed us, Sa'id bin Khalid narrated to me, from Abu Salamah bin 'Abd al-Rahman, from Abu Sa'id al-Khudri, from the Prophet (may Allah bless him and grant him peace), who said: "If a fly falls into the food of one of you, then submerge it, for in one of its wings is a disease and in the other is a cure."