Musnad Ahmad 8:33

Ya'qub and Sa'd narrated to us, they said: My father narrated to us, from his father, from his grandfather. Sa'd said: Ibrahim ibn 'Abd al-Rahman said: I heard Sa'd ibn Abi Waqqas saying: "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."