From Sahih Bukhari (Hadith 4054):
Narrated Sa`d bin Abi Waqqas:I saw Allah's Messenger (ﷺ) on the day of the battle of Uhud accompanied by two men fighting on his behalf. They were dressed in white and were fighting as bravely as possible. I had never seen them before, nor did I see them later on
Reference: Bukhari 4054