Hadithcore
Sahih al-Bukhari, 4171
sahih

Narrated Abu Qilaba:

that Thabit bin Ad-Dahhak had informed him that he was one of those who had given the Pledge of allegiance (of Al-Hudaibiya) beneath the Tree.

حَدَّثَنَا إِسْحَاقُ، حَدَّثَنَا يَحْيَى بْنُ صَالِحٍ، قَالَ حَدَّثَنَا مُعَاوِيَةُ ـ هُوَ ابْنُ سَلاَّمٍ ـ عَنْ يَحْيَى، عَنْ أَبِي قِلاَبَةَ، أَنَّ ثَابِتَ بْنَ الضَّحَّاكِ، أَخْبَرَهُ أَنَّهُ، بَايَعَ النَّبِيَّ صلى الله عليه وسلم تَحْتَ الشَّجَرَةِ‏.‏

References2 variants
In-Book Reference
Book 64, Hadith 213
USC-MSA web (English) reference
Vol. 5, Book 59, Hadith 489 (deprecated numbering scheme)