Musnad Abu Ya'la al-Mawsili 45:15

Abu Ma'mar al-Hudhali narrated to us, Hushaym narrated to us, Humayd informed us, from Anas, from Abu Talhah, who said: "The sword fell from me on the day of Badr when drowsiness overcame us." He says: "Allah says: 'When He covered you with drowsiness as a security from Him' [Al-Anfal: 11]."