Hadith 34:76
'Abd al-Rahim narrated to us, from Mujalid, from 'Amir, who said: The first person to pledge allegiance under the tree was Abu Sinan bin Wahb al-Asadi. He came to the Prophet, may Allah bless him and grant him peace, and said: "I pledge allegiance to you." He said: "Upon what do you pledge allegiance to me?" He said: "I pledge allegiance to you upon what is in your soul." Then the people pledged allegiance to him after that.
حدثنا عبد الرحيم عن مجالد عن عامر قال أول من بايع تحت الشجرة أبو سنان بن وهب الأسدي أتى النبي صلى الله عليه وسلم فقال أبايعك قال علام تبايعني قال أبايعك على ما في نفسك فبايعه الناس بعد