كتاب المغازى
64  
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Sahih al-Bukhari 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.

حَدَّثَنَا عَبْدُ الْعَزِيزِ بْنُ عَبْدِ اللَّهِ، حَدَّثَنَا إِبْرَاهِيمُ بْنُ سَعْدٍ، عَنْ أَبِيهِ، عَنْ جَدِّهِ، عَنْ سَعْدِ بْنِ أَبِي وَقَّاصٍ ـ رضى الله عنه ـ قَالَ رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَوْمَ أُحُدٍ، وَمَعَهُ رَجُلاَنِ يُقَاتِلاَنِ عَنْهُ، عَلَيْهِمَا ثِيَابٌ بِيضٌ، كَأَشَدِّ الْقِتَالِ، مَا رَأَيْتُهُمَا قَبْلُ وَلاَ بَعْدُ‏.‏
Reference : Sahih al-Bukhari 4054
In-book reference : Book 64, Hadith 100
USC-MSA web (English) reference : Vol. 5, Book 59, Hadith 384
  (deprecated numbering scheme)