Musannaf Ibn Abi Shayba 36:573

'Abdah ibn Sulayman narrated to us, from Mujalid, from 'Amir, who said: The first to pledge allegiance under the tree was Abu Sinan 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, "Upon what is in your soul." He said, "So he pledged allegiance to him." He said, "Then another man came to him and said, 'I pledge allegiance to you upon what Abu Sinan pledged allegiance to you.' So he pledged allegiance to him, then the people pledged allegiance to him."