Musannaf Ibn Abi Shayba 36:215

Muhammad ibn Bishr and Abu Usamah narrated to us, from Mis'ar, from Sa'd ibn Ibrahim, from his father, from Sa'd, 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, whom I had not seen before nor after.