Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
حَدَّثَنَا قَبِيصَةُ، حَدَّثَنَا سُفْيَانُ، عَنْ طَارِقٍ، قَالَ ذُكِرَتْ عِنْدَ سَعِيدِ بْنِ الْمُسَيَّبِ الشَّجَرَةُ فَضَحِكَ فَقَالَ أَخْبَرَنِي أَبِي وَكَانَ، شَهِدَهَا.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)
From Sahih al-Bukhari, whose reports the scholars accept as authentic.