Musnad Ahmad 8:30

Sulayman ibn Dawud al-Hashimi narrated to us, Ibrahim ibn Sa'd informed us, from his father, from his father, from Sa'd ibn Abi Waqqas, who said: "I saw on the right of the Messenger of Allah, may Allah bless him and grant him peace, and on his left on the day of Uhud, two men wearing white garments, fighting for him as fiercely as the most intense fighting. I had not seen them before, nor did I see them after."