Sahih al-Bukhari 4165
Narrated Tariq:
(The tree where the Ridwan Pledge of allegiance was taken by the Prophet) was mentioned before 
Sa`id bin Al-Musaiyab. On that he smiled and said, "My father informed me (about it) and he had 
witnessed it (i.e. the Pledge) ."
حَدَّثَنَا قَبِيصَةُ، حَدَّثَنَا سُفْيَانُ، عَنْ طَارِقٍ، قَالَ ذُكِرَتْ عِنْدَ سَعِيدِ بْنِ الْمُسَيَّبِ الشَّجَرَةُ فَضَحِكَ فَقَالَ أَخْبَرَنِي أَبِي وَكَانَ، شَهِدَهَا‏.‏