Musnad Ahmad 31:652

Yazid narrated to us, he said: Ibn Abi Dhi'b narrated to us, from Sa'id ibn Khalid, he said: I entered upon Abu Salamah, and he brought us butter and a lump of curd. A fly fell into the food, and Abu Salamah began to submerge it into it with his finger. I said: "O uncle, what are you doing?" He said: "Abu Sa'id al-Khudri narrated to me, from the Messenger of Allah, peace and blessings of Allah be upon him, that he said: 'Indeed, one of the two wings of a fly contains poison, and the other contains a cure. So if it falls into food, submerge it, for it presents the poison and delays the cure.'"