Musnad Ahmad 30:2046

Sulayman narrated to us, Isma'il informed us, 'Utbah ibn Muslim, the freed slave of Banu Taym, informed me, from 'Ubayd ibn Hunayn, the freed slave of Banu Zurayq, from Abu Hurayrah, that the Prophet, peace and blessings of Allah be upon him, said: "If a fly falls into the drink of one of you, let him submerge it entirely, then let him cast it away. For indeed, in one of its wings is a cure, and in the other is a disease."