Sahih Bukhari Volume 5, Book 64, Hadith 4165

حَدَّثَنَا مَحْمُودٌ، حَدَّثَنَا عُبَيْدُ اللَّهِ، عَنْ إِسْرَائِيلَ، عَنْ طَارِقِ بْنِ عَبْدِ الرَّحْمَنِ، قَالَ انْطَلَقْتُ حَاجًّا فَمَرَرْتُ بِقَوْمٍ يُصَلُّونَ قُلْتُ مَا هَذَا الْمَسْجِدُ قَالُوا هَذِهِ الشَّجَرَةُ، حَيْثُ بَايَعَ رَسُولُ اللَّهِ صلى الله عليه وسلم بَيْعَةَ الرُّضْوَانِ‏.‏ فَأَتَيْتُ سَعِيدَ بْنَ الْمُسَيَّبِ فَأَخْبَرْتُهُ فَقَالَ سَعِيدٌ حَدَّثَنِي أَبِي أَنَّهُ كَانَ فِيمَنْ بَايَعَ رَسُولَ اللَّهِ صلى الله عليه وسلم تَحْتَ الشَّجَرَةِ، قَالَ فَلَمَّا خَرَجْنَا مِنَ الْعَامِ الْمُقْبِلِ نَسِينَاهَا، فَلَمْ نَقْدِرْ عَلَيْهَا‏.‏ فَقَالَ سَعِيدٌ إِنَّ أَصْحَابَ مُحَمَّدٍ صلى الله عليه وسلم لَمْ يَعْلَمُوهَا وَعَلِمْتُمُوهَا أَنْتُمْ، فَأَنْتُمْ أَعْلَمُ‏

Narrated By Tariq bin 'Abdur-Rahman : When I set out for Hajj, I passed by some people offering a prayer, I asked, "What is this mosque?" They said, "This is the Tree where Allah's Apostle took the Ar-Ridwan Pledge of allegiance. Then I went to Sa'id bin Musaiyab and informed him about it. Said said, "My father said that he was amongst those who had given the Pledge of allegiance to Allah's Apostle beneath the Tree. He (i.e. my father) said, "When we set out the following year, we forgot the Tree and were unable to recognize it. "Then Said said (perhaps ironically) "The companions of the Prophet could not recognize it; nevertheless, you do recognize it; therefore you have a better knowledge."