Musnad Ahmad 502:1

'Abd al-Razzaq narrated to us, Ibn Jurayj informed us, 'Abdullah ibn 'Uthman ibn Khuthaym informed me, that Muhammad ibn al-Aswad ibn Khalaf informed him that his father, al-Aswad, came to the Prophet (peace and blessings of Allah be upon him) while he was accepting the pledge of allegiance from the people on the day of the Conquest. He said: "He sat at Qarn Masfalah, and he accepted the pledge of allegiance from the people upon Islam and the Testimony (Shahadah)." I said: "And what is the Testimony?" He said: "Muhammad ibn al-Aswad—meaning ibn Khalaf—informed me that he accepted their pledge of allegiance upon belief in Allah, and the testimony that there is no god but Allah, and that Muhammad is His servant and His Messenger."