Musnad Abu Ya'la al-Mawsili 148:40

Abu Sa’id al-Ashajj narrated to us, ‘Uqbah ibn Khalid narrated to us, ‘Anbasah al-Qass narrated to me, from Hanzalah, 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 days, and all flies are in the Fire."