Musnad Abu Ya'la al-Mawsili 147:50

Shayban ibn Farrukh narrated to us, [saying] Sukeen ibn Abd al-Aziz narrated to us, from his father, from Anas ibn Malik, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The lifespan of a fly is forty nights, and all flies are in the Fire, except for the bee."