Sahih Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Volume: 005 / Book Number: 059 / Hadith Number: 483
Narrated Tariq:
(The tree where the Ridwan Pledge of allegiance was taken by the Prophet) was mentioned before Said bin Al-Musaiyab. On that he smiled and said, "My father informed me (about it) and he had witnessed it (i.e. the Pledge) ."