Musnad Ahmad

It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: If a fly falls into the vessel of any one of you, then let him remember that in one wing of the fly there is healing and in the other there is disease, and it protects itself with the wing that has the disease (it puts that wing in the vessel first). Therefore, he should fully immerse the fly in it (then using it is up to his discretion).