Sahih Muslim
The Book Pertaining to the Excellent Qualities of the Holy Prophet (may Peace be upon them) and His Companions (Kitab Al-Fadail)
Book Number: 030 / Hadith Number: 5714
Sa'd b. Abu Waqqas reported: I saw on the right side of Allah's Messenger (may peace be upon him) and on his left side two persons with white clothes on the Day of Uhtid fighting a desperate fight, and I saw them neither before nor after that.