Musnad Ahmad 31:201

Yahya narrated to us, Ibn Abi Dhi'b narrated to us, he said: Sa'id ibn Khalid narrated to me, from Abu Salamah, from Abu Sa'id al-Khudri, from the Prophet, peace and blessings of Allah be upon him, who said: "If a fly falls into the food of one of you, then submerge it."