Musnad Ahmad 816:1

Waki' narrated to us, 'Abd al-Majid Abu 'Amr narrated to me, al-'Ada' ibn Khalid ibn Hawdhah narrated to me, he said: "I saw the Messenger of Allah, peace and blessings of Allah be upon him, addressing the people on the day of 'Arafah while upon a camel, standing in the two stirrups."