Musnad Ahmad 8:92

Narrated to us Muhammad ibn Ubayd, narrated to us Mis'ar, from Sa'd ibn Ibrahim, from his father, from Sa'd ibn Abi Waqqas, who said: "I saw on the right of the Messenger of Allah, peace and blessings of Allah be upon him, and on his left, on the day of Uhud, two men wearing white garments, whom I had not seen before nor after."