Sahih Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Volume: 005 / Book Number: 059 / Hadith Number: 384
Narrated Sad bin Abi Waqqas:
I saw Allah's Apostle 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.