Musnad Ahmad 31:910

'Affan and Hasan narrated to us, they said: Hammad narrated to us, from Bishr ibn Harb, from Abu Sa'id al-Khudri, that he said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to supplicate at 'Arafah." Hasan said: "And he would raise his hands like this, placing their backs upward and their palms downward." Hammad described it, and Hammad raised his hands and his palms toward the direction of the ground.