Musnad Ahmad 31:106

Rawh narrated to us, Hammad narrated to us, from Bishr ibn Harb, from Abu Sa’id al-Khudri, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) was standing at Arafah supplicating like this, and he raised his hands level with his chest, and he placed the palms of his hands facing toward the ground."