Musnad Ahmad 5:690

Abu Nu'aym narrated to us, Mis'ar narrated to us, from Abu 'Awn, from Abu Salih al-Hanafi, from Ali, who said: "It was said to Ali and to Abu Bakr on the day of Badr: 'With one of you is Jibril, and with the other is Mika'il, and Israfil is a mighty angel who witnesses the fighting'—or he said: 'witnesses the ranks.'"