Thabit bin Dahhak informed him that he pledged allegiance to the Prophet ﷺ under the tree. The hadith continues.
أن ثابت بن الضحاك أخبره أنه بايع النبي ﷺ تحت الشجرة. الحديث.
Bukhari reported it in the Book of Military Expeditions (4171), and Muslim in the Book of Faith (110:176), both through Muawiya bin Salam bin Abu Salam Dimashqi, from Yahya bin Abu Kathir, that Abu Qilabah informed him, that Thabit bin Dahhak informed him. He then narrated it.