Narrated Al-Musaiyab:

I met Al-Bara bin `Azib and said (to him). "May you live prosperously! You enjoyed the company of the Prophet (ﷺ) and gave him the Pledge of allegiance (of Al-Hudaibiya) under the Tree." On that, Al- Bara' said, "O my nephew! You do not know what we have done after him (i.e. his death).

حَدَّثَنِي أَحْمَدُ بْنُ إِشْكَابٍ، حَدَّثَنَا مُحَمَّدُ بْنُ فُضَيْلٍ، عَنِ الْعَلاَءِ بْنِ الْمُسَيَّبِ، عَنْ أَبِيهِ، قَالَ لَقِيتُ الْبَرَاءَ بْنَ عَازِبٍ ـ رضى الله عنهما ـ فَقُلْتُ طُوبَى لَكَ صَحِبْتَ النَّبِيَّ صلى الله عليه وسلم وَبَايَعْتَهُ تَحْتَ الشَّجَرَةِ‏.‏ فَقَالَ يَا ابْنَ أَخِي إِنَّكَ لاَ تَدْرِي مَا أَحْدَثْنَا بَعْدَهُ‏.‏
Reference : Sahih al-Bukhari 4170
In-book reference : Book 64, Hadith 212
USC-MSA web (English) reference : Vol. 5, Book 59, Hadith 488
  (deprecated numbering scheme)