Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Bukhari :: Book 5 :: Volume 59 :: Hadith 480
Narrated Said bin Al-Musaiyab:
That his father said, "I saw the Tree (of the Ar-Ridwan Pledge of allegiance and when I returned to it later, I was not able to recognize it. (The sub--narrator MahmiJd said, Al-Musaiyab said, 'Then; forgot it (i.e., the Tree).)"