From Sahih Bukhari (Hadith 4162):
Narrated Sa`id 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 Mahmud said, Al-Musaiyab said, 'Then I forgot it (i.e., the Tree)
Reference: Bukhari 4162